Optical information recording/ reproducing apparatus and optical information recording and reproducing method

ABSTRACT

An optical information recording/reproducing device includes a liquid optical element containing a liquid crystal polymer layer in an optical head. A liquid crystal optical element drive unit drives a liquid crystal optical element having a first pattern electrode divided into a plurality of region at one side of the liquid crystal polymer layer in the optical axis direction. The first pattern electrode includes a first region arranged to surround the optical axis and second to ninth regions arranged outside the first regions in such a manner that the circumference is divided eight portions. The liquid crystal optical element drive unit applies an effective voltage to each region. The voltage applied to the respective regions of the pattern region of the liquid crystal optical element is decided in a short time so as to optimize the quality of the reproduction signal.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. Ser. No.12/092,924, filed May 7, 2008, which is a U.S.C. 371 application ofPCT/JP2006/322164 filed on Nov. 7, 2006, which claims the benefit ofpriority of Japanese Application No. 2005-323920 filed Nov. 8, 2005 andJapanese Application No. 2006-298453 filed Nov. 2, 2006, the entirecontents of which are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to an optical informationrecording/reproducing apparatus and an optical informationrecording/reproducing method for performing recording and reproductionon an optical recording medium.

BACKGROUND ART

The recording density of an optical recording medium is inverselyproportional to the square of the diameter of the focused spot formed onthe optical recording medium by an optical head incorporated within anoptical information recording/reproducing apparatus that performsrecording or reproduction on the optical recording medium. That is, asmaller focused spot diameter results in higher recording density. Thefocused spot diameter is proportional to the wavelength of the lightsource in the optical head and inversely proportional to the numericaperture of the objective lens. That is, a shorter wavelength of thelight source and higher numeric aperture of the objective lens result ina smaller focused spot diameter.

An optical system of an optical head suffers from various types ofaberration, such as astigmatism, coma aberration, and sphericalaberration, due to manufacturing error and adjustment error of opticalcomponents. For example, misalignment between the center of theincidence surface of the objective lens and the center of the exitsurface thereof causes astigmatism and coma aberration, and deviation ofthe spacing between the incidence surface and exit surface of theobjective lens from design causes spherical aberration. The occurrenceof the various types of aberration in the optical system of the opticalhead results in a disturbed shape of the focused spot and deterioratedrecording/reproducing characteristics. The astigmatism, coma aberration,and spherical aberration are inversely proportional to the wavelength ofthe light source, and proportional to the square, cube, andfourth-power, respectively, of the numeric aperture of the objectivelens. Thus, a shorter wavelength of the light source and higher numericaperture of the objective lens result in a narrower margin of thevarious types of aberrations for the recording/reproducingcharacteristics. Therefore, an optical information recording/reproducingapparatus provided with a light source of a shortened wavelength and anobjective lens of an increased numeric aperture for improved recordingdensity requires correction of the various types of aberration occurringin the optical system of the optical head in order to preventdeterioration in the recording/reproducing characteristics.

Known as conventional optical heads capable of correcting various typesof aberration are optical heads provided with a liquid crystal opticalelement for correcting various types of aberration. Among them, anexample of the conventional optical heads provided with a liquid crystaloptical element for correcting astigmatism is described in JapaneseLaid-Open Patent Application No. JP-A 2000-40249. FIG. 28 shows theconfiguration of the optical head disclosed in Japanese Laid-Open PatentApplication No. JP-A 2000-40249. This optical head includes: asemiconductor laser 26, a polarizing beam splitter 27, a liquid crystaloptical element 28, a quarter-wave plate 29, an objective lens 30, aconvex lens 32, and a photo-detector 33. Light emitted from thesemiconductor laser 26 serving as a light source is made incident asP-polarized light on the polarizing beam splitter 27, and is transmittedtherethrough almost completely, and then outputted to the liquid crystaloptical element 28. The liquid crystal optical element 28 transmits theincident light and outputs it to the quarter-wave plate 29. Thequarter-wave plate 29 converts the transmitted light from linearpolarized light into circular polarized light. The light transmittedthrough the quarter-wave plate 29 is focused on a disk 31 serving as anoptical recording medium by the objective lens 30. The light reflectedon the disk 31 is transmitted backward through the objective lens 30,and made incident on the quarter-wave plate 29. The quarter-wave plate29 converts the transmitted light from the circular polarized light intolinear polarized light whose polarization direction is orthogonal tothat on the forward path. The light converted into the linear polarizedlight is transmitted backward through the liquid crystal optical element28, and is made incident as S-polarized light on the polarizing beamsplitter 27. The polarizing beam splitter 27 reflects the incident lightalmost completely and outputs it to the convex lens 32. The lighttransmitted through the convex lens 32 is received by the photo-detector33.

The liquid crystal optical element 28 is structured to have liquidcrystal polymer sandwiched between two substrates. A pattern electrode34 is formed on a surface of one of the substrates on the liquid crystalpolymer side, and an entire surface electrode is formed on a surface ofthe other substrate on the liquid crystal polymer side. FIG. 29 is aplan view of the pattern electrode 34 of the liquid crystal opticalelement 28. The pattern electrode 34 is divided into nine regions.Specifically, the pattern electrode 34 is divided into: a circularregion 35 a with the optical axis as center; regions 35 b to 35 i whichare located outside of the region 35 a and divided by four straightlines passing through the optical axis in units of 45 degrees inaccordance with the angle around the optical axis. A dotted line in thefigure indicates the effective diameter of the objective lens 30.

FIG. 30 shows relationship between the regions of the pattern electrode34 of the liquid crystal optical element 28 and voltages respectivelyapplied to these regions. Nine drive patterns, drive pattern A to drivepattern I, are available for the liquid crystal optical element 28. Asshown in FIG. 30, selected one of drive voltages Va, Vb, and Vc isapplied to each of the regions 35 a to 35 i, in accordance with therespective drive pattern. Here, Va>Vc>Vb, and Va−Vc=Vc−Vb=V. Thetransmitted light through the region(s) fed with the drive voltage Va isadvanced in phase with respect to the transmitted light through theregion(s) fed with the drive voltage Vc. The transmitted light throughthe region(s) fed with the drive voltage Vb is delayed in phase withrespect to the transmitted light through the region(s) fed with thedrive voltage Vc.

The drive pattern A advances the phase of the light transmitted throughthe regions 35 c, 35 d, 35 g, and 35 h with respect to the lighttransmitted through the region 35 a, and delays the phase the lighttransmitted through the regions 35 b, 35 e, 35 f, and 35 i with respectto the light transmitted through the region 35 a. On the other hand, thedrive pattern E delays the phase of the light transmitted through theregions 35 c, 35 d, 35 g, and 35 h with respect to the light transmittedthrough the region 35 a, and advances the phase of the light transmittedthrough the regions 35 b, 35 e, 35 f, and 35 i with respect to the lighttransmitted through the region 35 a. Therefore, the use of the drivepatterns A or E successfully provides correction of the astigmatismbetween the 0° direction and the 90° direction. The sign of correctableastigmatism is opposite between the drive patterns A and E.

The drive pattern C advances the phase of the light transmitted throughthe regions 35 d, 35 e, 35 h, and 35 i with respect to the lighttransmitted through the region 35 a, and delays the phase of the lighttransmitted through the regions 35 b, 35 c, 35 f, and 35 g with respectto the light transmitted through the region 35 a. On the other hand, thedrive pattern G advances the phase of the light transmitted through theregions 35 d, 35 e, 35 h, and 35 i with respect to the light transmittedthrough the region 35 a, and delays the phase of the light transmittedthrough the regions 35 b, 35 c, 35 f, and 35 g with respect to the lighttransmitted through the region 35 a. Therefore, the drive patterns C andG provide correction of the astigmatism between the 45° direction andthe 135° direction. The sign of correctable astigmatism is oppositebetween the drive patterns C and G.

The drive pattern D advances the phase of the light transmitted throughthe regions 35 e and 35 i with respect to the light transmitted throughthe region 35 a, and delays the phase of the light transmitted throughthe regions 35 c and 35 g with respect to the light transmitted throughthe region 35 a. On the other hand, the drive pattern H delays the phaseof the light transmitted through the regions 35 e and 35 i with respectto the light transmitted through the region 35 a, and advances the phaseof the light transmitted through the regions 35 c and 35 g with respectto the light transmitted through the region 35 a. Therefore, the drivepatterns D and H provide correction of the astigmatism between 22.5°direction and 112.5° direction. The sign of correctable astigmatism isopposite between the drive patterns D and H.

The drive pattern B advances the phase of the light transmitted throughthe regions 35 d and 35 h with respect to the light transmitted throughthe region 35 a, and delays the phase of the light transmitted throughthe regions 35 b and 35 f with respect to the light transmitted throughthe region 35 a. On the other hand, the drive pattern F delays the phaseof the light transmitted through the regions 35 d and 35 h with respectto the light transmitted through the region 35 a, and delays the phaseof the light transmitted through the regions 35 b and 35 f with respectto the light transmitted through the region 35 a. Therefore, the drivepatterns D and H provide correction of the astigmatism between 67.5°direction and 157.5° direction. The sign of correctable astigmatism isopposite between the drive patterns B and F.

The absolute amount of astigmatism correctable with the drive patterns Ato H increases with the increase in the value of the voltage V. Itshould be noted that the drive pattern I does not provide astigmatismcorrection.

The correction of astigmatism with the liquid crystal optical element 28requires selecting any of the drive patterns A to I in accordance withthe direction of astigmatism to be corrected, and determining the levelof voltage V in accordance with the amount of astigmatism to becorrected, so that the quality evaluation index of the reproduced signalfrom the optical recording medium is best improved. Japanese Laid OpenPatent Application No. JP-A 2000-40249 discloses two methods as methodsof determining which of the drive patterns A to I is to be used anddetermining the value of voltage V.

The first method involves measuring the jitter of the reproduced signaland selecting the combination of the drive pattern and the value of thevoltage V so that the jitter is minimized. Eight drive patterns A to Hare preliminary prepared for the drive pattern, and about 32 types ofvoltage values are previously prepared for the voltage V. For all thecombinations, the jitter of the reproduced signal is measured, and thecombination of the drive pattern and the voltage V is selected so as tominimize the jitter.

A second method involves measuring the amplitude of the reproducedsignal and selecting the drive pattern so that the jitter is minimized.Eight drive patterns A to H are previously prepared for the drivepattern, about 16 types of voltage values are previously prepared forthe voltage V. First, the voltage V is fixed at any one of about the 16types of voltage values, and the amplitude of the reproduced signal,which is one of the quality evaluation indexes of the reproduced signal,is measured for all the eight drive patterns. The drive pattern isselected so that the measured amplitude of the reproduced signal isminimized. Next, the amplitude of the reproduced signal is measured forall the about 16 types of voltage V with the selected one of the eighttypes of drive patterns fixed. From among them, the voltage V isselected so that the amplitude of the reproduced signal is maximized.

The first method allows selecting the optimum combination of the drivepattern and voltage V which offers the best quality evaluation index ofthe reproduced signal. However, the first method requires long time toselect the combination of the drive pattern and voltage V. On the otherhand, the second method allows selecting the combination of the drivepattern and voltage V in short time. However, the second method does notnecessary select the optimum combination of the drive pattern andvoltage V which provides the best quality evaluation index of thereproduced signal.

In connection with the above, Japanese Laid Open Patent Application No.JP-A 2001-273663 discloses an aberration correction device. Theconventional aberration correction device corrects aberration occurringon the optical path of an optical system that irradiates an optical beamto a recording medium and then guides the optical beam reflected by therecording medium. This aberration correction device includes: a liquidcrystal unit provided with a first electrode layer including a pluralityof divided electrodes electrically separated from each other in the sameplane, a second electrode layer, and a liquid crystal element which isprovided between the first and second electrode layers to cause a phasechange in the light passing therethrough in accordance with the appliedelectric field; a detector which receives a reflected optical beamtraveling through the liquid crystal unit to generate a detectionsignal; a voltage generator which generates voltages respectivelyapplied to the plurality of divided electrodes; and a controller whichcontrols aberration correction by, with the voltage applied to thepredetermined divided electrode of the first electrode layer beingdefined as a reference voltage, changing the voltages applied to theother divided electrodes. The controller defines the reference voltagebased on the change in the magnitude of the detection signal for thechange of the voltages respectively applied to the plurality of dividedelectrodes.

Moreover, Japanese Laid Open Patent Application No. JP-A2002-14314discloses an optical recording/reproducing device. The conventionaloptical recording/reproducing device includes: a voltage applicationelectrode having a segment electrode part composed of a plurality ofsegment electrodes, a voltage control part formed of a conductivematerial and generating voltages applied to the plurality of segmentelectrodes by dividing an externally applied voltage by resistors ofconductive material, a conduction part connecting together the segmentelectrode part and the voltage control part, and an insulation partpreventing short-circuit between the conduction part; an oppositeelectrode arranged in substantially parallel to the voltage applicationelectrode and opposed to the voltage application electrode; and a phasechange layer formed of a phase changing material arranged between thevoltage application electrode and the opposite electrode. The phase oflight incident on the phase change layer is changed by changing thedifference in the voltage between the plurality of segment electrodesand the opposite electrode.

Moreover, Japanese Laid-Open Patent Application No. JP-A2003-338070discloses an optical head device. In this conventional example, theoptical head device includes: a light source; an objective lens forfocusing the light emitted from the light source onto an opticalrecording medium; a phase correction element provided between the lightsource and the objective lens to change the wavefronts of the emittedlight; and control voltage generating means outputting awavefront-changing voltage to the phase correction element. The phasecorrection element includes: a pair of transparent substrates withtransparent electrodes formed on the surfaces thereof; and a liquidcrystal layer sandwiched between the transparent substrates. Formed onthe surface of at least one of the transparent substrates are: a comaaberration correction electrode which is a transparent electrode forcorrecting coma aberration or a spherical aberration correctionelectrode which is a transparent electrode for correcting sphericalaberration; and an astigmatism correction electrode which is atransparent electrode for correcting astigmatism. Each of thetransparent electrodes is divided into several segments.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an optical informationrecording/reproducing apparatus and an optical informationrecording/reproducing method for determining voltages to be applied torespective regions of a pattern electrode of a liquid crystal element inshort time so that the quality of the reproduced signal is bestimproved.

In an aspect of the present invention, an optical informationrecording/reproducing apparatus is provided with an optical head and aliquid crystal optical element drive unit. The optical head includes alight source, an objective lens focusing an emitted light emitted fromthe light source on an optical recording medium, a photo-detectorreceiving a reflected light generated by the emitted light beingreflected by the optical recording medium, light splitting means ofsplitting forward light directed from the light source to the objectivelens and backward light directed from the objective lens to thephoto-detector, and a liquid crystal optical element which is providedin a light path of the forward light and includes a liquid crystalpolymer layer extending perpendicularly to an optical axis. Said liquidcrystal optical element includes a first pattern electrode provided onone side of the liquid crystal polymer layer in the optical axisdirection and divided into a plurality of regions, and the liquidcrystal optical element drive unit drives this liquid crystal opticalelement. The first pattern electrode includes: a first region providedto surround the optical axis, a set of eight partition regions providedoutside the first regions and defined so as to divide a circumferenceinto eight segments with respect to the optical axis. This set of eightpartition regions are referred to as second to ninth regions in order.The second to ninth regions are preferably provided to in such a manneras to divide the circumference around the optical axis into eight equalsegments. The liquid crystal element drive unit applies voltages to therespective regions of the first pattern electrode as follows: The liquidcrystal element drive unit applies a first effective voltage to thefirst region, a second effective voltage to the second and sixthregions, a third effective voltage to the third and seventh regions, afourth effective voltage to the fourth and eighth regions, and a fiftheffective voltage to the fifth and ninth regions. These applied voltageshold the following relation: The average of the second and fourtheffective voltages and the average of the third and fifth effectivevoltages are equal to the first effective voltage.

In the present invention, the first pattern electrode may furtherinclude a second set of eight partition regions which are providedoutside the second to ninth regions and defined so as to divide acircumference into eight segments. The second set of eight partitionregions are refereed to as tenth to seventeenth regions in order. Thetenth to seventeenth regions are preferably arranged to divide thecircumference into eight equal segments. The liquid crystal elementdriver unit further drives the liquid crystal optical element asfollows: The liquid crystal element driver unit applies a sixtheffective voltage to the tenth and fourteenth regions, a seventheffective voltage to the eleventh and fifteenth regions, an eightheffective voltage to the twelfth and sixteenth regions, and a nintheffective voltage to the thirteenth and seventeenth regions.

These applied voltages satisfy the relation as follows: The average ofthe sixth and eighth effective voltages and the average of the seventhand ninth voltages are equal to the first effective voltage. thedifference between the sixth and first effective voltages is K times thedifference between the second and the first effective voltages, where Kis a constant larger than one. The difference between the seventh andfirst effective voltages is K times the difference between the third andfirst effective voltages. The difference between the eighth and firsteffective voltages is K times the difference between the fourth andfirst effective voltages. The difference between the ninth and firsteffective voltages is K times the difference between the fifth and firsteffective voltages.

In the present invention, the liquid crystal optical element may furtherincludes a second pattern electrode, provided on another side of theliquid crystal polymer layer in the optical axis direction, positionedopposed to the first pattern electrode, and divided into a plurality ofregions. The second pattern electrode is divided into eighteenth totwenty-second regions. The eighteenth and nineteenth regions areprovided apart from each other in an island structure at positions insymmetry with respect to a straight line passing through the opticalaxis and directed in a predetermined direction. The twentieth region isprovided outside the eighteenth and nineteenth regions to surround theeighteenth and nineteenth regions. The twenty-first and twenty-secondregions are provided outside the twentieth region approximately insymmetry with the straight line on a same side of the eighteenth andnineteenth regions with respect to the straight line. The liquid crystalelement drive unit further applies a tenth effective voltage to thetwentieth region, an eleventh effective voltage to the eighteenth andtwenty-second regions, and a twelfth effective voltage to the nineteenthand twenty first regions. The average of the eleventh and twelftheffective voltages is equal to the tenth effective voltage.

In the present invention, the liquid crystal optical element may furtherinclude a third pattern electrode provided on another side of the liquidcrystal polymer layer in the optical axis direction, positioned opposedto the first pattern electrode, and divided into eighteenth andtwenty-second regions arranged in order from inside to outside tosurround the optical axis. In this case, the liquid crystal elementdrive unit applies a thirteenth effective voltage to the nineteenth andtwenty-first regions, a fourteenth effective voltage to the twentiethregion, and a fifteenth effective voltage to the eighteenth andtwenty-second regions. The average of the fourteenth and fifteentheffective voltages is equal to the thirteenth effective voltage.

In another aspect of the present invention, an optical informationrecording/reproducing method includes a driving step, a reproductionstep, and a control step. In the driving step, a liquid crystal opticalelement is driven which is disposed in an optical path of forward lightwithin an optical head, provided with a liquid crystal polymer layerextending perpendicularly to an optical axis and including a firstpattern electrode positioned on one side of the liquid crystal polymerlayer in the optical axis direction. In the reproducing step, areproduced signal is generated based on backward light reflected by anoptical recording medium. In the control step, the drive of the liquidcrystal optical element in the driving step is controlled so that aquality evaluation index of the reproduced signal is best improved. Thefirst pattern electrode includes: a first region provided to surroundthe optical axis, a set of eight partition regions provided outside thefirst regions and defined so as to divide a circumference into eightsegments with respect to the optical axis. The set of eight partitionregions are referred to as second to ninth regions in order. The secondto ninth regions are preferably arranged to divide the circumferencearound the optical into eight equal segments. The driving step includes:a step of applying an effective voltage of V1 to the first region, astep of applying an effective voltage to the second and sixth regions; astep of applying an effective voltage to the third and seventh regions;a step of applying an effective voltage to the fourth and eighthregions; and a step of applying an effective voltage of V1+Vα−Vβ to thefifth and ninth regions.

In the present invention, the first pattern electrode may furtherinclude: a second set of eight partition regions which are providedoutside the second to ninth regions and defined so as to divide acircumference into eight segments. The second set of eight partitionregions are referred to as tenth to seventeenth regions in order. Thetenth to seventeenth regions are preferably arranged to divide thecircumference around the optical axis into eight equal segments. Thedriving step further includes: a step of applying an effective voltageto the tenth and fourteenth regions, a step of applying an effectivevoltage to the eleventh and fifteenth regions, a step of applying aneffective voltage to the twelfth and sixteenth regions; and a step ofapplying an effective voltage to the thirteenth and seventeenth regions,where K is a constant larger than one.

The first pattern electrode may include: a first region provided tosurround the optical axis, a set of eight partition regions providedoutside the first regions and defined so as to divide a circumferenceinto eight segments with respect to the optical axis. This set of eightpartition regions are referred to as second to ninth regions in order.The second to ninth regions are preferably provided to in such a manneras to divide the circumference around the optical axis into eight equalsegments. In this case, the driving step includes: a step of applying afirst effective voltage to the first region, a step of applying a secondeffective voltage to the second and sixth regions, a step of applying athird effective voltage to the third and seventh regions, a step ofapplying a fourth effective voltage to the fourth and eighth regions,and a step of applying a fifth effective voltage to the fifth and ninthregions.

In the present invention, the first pattern electrode may furtherinclude a second set of eight partition regions which are providedoutside the second to ninth regions and defined so as to divide acircumference into eight segments. The second set of eight partitionregions are referred to as tenth to seventeenth regions in order. Thetenth to seventeenth region are preferably arrange so as to divide thecircumference around the optical axis into eight equal segments. In thiscase, the driving step further includes: a step of applying an effectivevoltage to the tenth and fourteenth regions, a step of applying aneffective voltage to the eleventh and fifteenth regions, a step ofapplying an effective voltage to the twelfth and sixteenth regions, anda step of applying an effective voltage of V1−K·Vβ to the thirteenth andseventeenth regions, where K is a constant larger than one.

In the present invention, the control step may include: a step ofdetermining an optimum value of the voltage Vα with the voltage Vβ fixedat a predetermined value so that the quality evaluation index is bestimproved, and a step of determining an optimum value of the voltage Vβwith the voltage Vα fixed at a predetermined value so that the qualityevaluation index is best improved.

In the present invention, the liquid crystal optical element may furtherincludes a second pattern electrode provided on another side of theliquid crystal polymer layer in the optical axis direction, positionedopposed to the first pattern electrode, and divided into a plurality ofregions. The second pattern electrode includes: eighteenth andnineteenth regions provided apart from each other in an island structureat positions in symmetry with respect to a straight line passing throughthe optical axis and directed in a predetermined direction, a twentiethregion provided outside the eighteenth and nineteenth regions tosurround the eighteenth and nineteenth regions, and twenty-first andtwenty-second regions provided outside the twentieth regionapproximately in symmetry with the straight line on a same side of theeighteenth and nineteenth regions with respect to the straight line. Thedriving step further includes a step of applying an effective voltage ofV2 to the twentieth region, a step of an effective voltage of V2−VΥ tothe eighteenth and twenty-second regions, and a step of applying aneffective voltage of V2−VΥ to the nineteenth and twenty first regions,where V2 is a second reference voltage value different from said firstreference value, and VΥ is a third voltage value.

In the present invention, the control step further includes a step ofdetermining an optimum value of the voltage VΥ with the voltages Vα andVβ fixed at predetermined values, so that the quality evaluation indexis best improved.

In the present invention, the liquid crystal optical element may furtherincludes a third pattern electrode provided on another side of theliquid crystal polymer layer in the optical axis direction, positionedopposed to the first pattern electrode, and divided into eighteenth andtwenty-second regions provided in order from inside to outside tosurround the optical axis. The driving step further includes a step ofapplying an effective voltage of V3 to the nineteenth and twenty-firstregions, a step of applying an effective voltage to the twentiethregion, and a step of applying an effective voltage of V3+Vδ to theeighteenth and twenty-second regions.

In the present invention, the control step may further include a step ofdetermining an optimum value of the voltage Vδ with the voltages Vα andVβ fixed at predetermined values, so that the quality evaluation indexis best improved. Further, the quality evaluation index is preferablyany of an amplitude, a jitter, a PRSNR, and an error rate of areproduced signal.

Wave aberration caused by astigmatism of an arbitrary direction andamount is expressed by a quadratic function of X and Y, with anassumption that X and Y axes are defined in the radial direction andtangential direction of an optical recording medium, respectively. Ageneral form of the quadratic function of X and Y is given asAX²+2BXY+CY² (where A, B, and C are constants). Therefore, correction ofthe aberration for which the wave aberration is expressed by thisformula allows correction of the astigmatism of an arbitrary directionand amount. This formula can be modified into (A+C) (X²+Y²)/2+(A−C)(X²−Y²)/2+2BXY, where (X²+Y²) denotes wave aberration caused by defocusaberration, (X²−Y²) denotes wave aberration caused by astigmatismbetween the 0° and 90° directions, and 2XY denotes wave aberrationcaused by astigmatism between the 45° and 135° directions.

The defocus aberration is automatically corrected by focus servo, andtherefore the correction of the astigmatism of an arbitrary directionand amount can be achieved by simultaneously correct the astigmatismbetween the 0° and 90° directions and the astigmatism between the 45°and 135° directions with the liquid crystal optical element. Here,(A−C)/2 and B are independent from each other, where (A−C)/2 is thecoefficient of (X²−Y²) and B is the coefficient of 2XY. Thus, a changein the amount of one of the astigmatism between the 0° and 90°directions and the astigmatism between the 45° and 135° directions doesnot result in a change in the amount of the other astigmatism.

In the optical information recording/reproducing apparatus and theoptical information recording/reproducing method according to thepresent invention, a change in the voltage Vα causes a change in thecorrection amount of astigmatism between 0° and 90° directions, while achange in the voltage Vβ causes a change in the correction amount ofastigmatism between 45° and 135° directions. Therefore, determining thevoltage Vα with the voltage Vβ fixed so that the quality evaluationindex of the reproduced signal is best improved allows obtaining theoptimum correction amount for the astigmatism between the 0° and 90°directions so that remaining wave aberration caused by this aberrationis minimized. Moreover, determining the voltage Vβ with the voltage Vαfixed so that the quality evaluation index of a reproduced signal isbest improved allows obtaining the optimum correction amount for theastigmatism between the 45° and 135° directions such that remaining waveaberration caused by this aberration is minimized.

A change in the amount of one of the astigmatism between the 0° and 90°directions and the astigmatism between 45° and 135° directions does notresult in a change in the amount of the other astigmatism. Thus, thevoltage Vα such that the quality evaluation index of the reproducedsignal is best improved (the optimum voltage Vα0) does not depend on thevoltage Vβ, and the voltage Vβ such that the quality evaluation index ofthe reproduced signal is best improved (optimum voltage Vβ0) does notdepend on the voltage Vα. Therefore, whichever of the step fordetermining the optimum voltage Vα0 with the Vβ fixed and the step fordetermining the optimum voltage Vβ0 with the Vα fixed is carried outfirst, the same combination of the optimum voltages Vα0 and Vβ0 isdetermined.

After the determination of the optimum voltages Vα0 and Vβ, theaforementioned drive voltages are respectively applied to the regions ofthe pattern electrode of the liquid crystal optical element on the basisof this combination. This allows simultaneously minimizing the remainingwave aberration caused by the astigmatism between 0° and 90° directionsand the remaining wave aberration caused by the astigmatism between the45° and 135° directions. That is, the correction of the astigmatism ofan arbitrary direction and amount is achieved. In this case, the qualityof the reproduced signal from the optical recording medium is bestimproved. Furthermore, in determining the combination of the optimumvoltages Vα0 and Vβ0, the quality evaluation index of the reproducedsignal is only measured for all the voltages Vα with the voltage Vβfixed and for all the voltages Vβ with the voltage Vβ fixed, not for allthe combinations of the voltages Vα and the voltage Vβ. Thus, thecombination of the optimum voltages Vα0 and Vβ0 is determined in a shorttime.

The present invention allows providing an optical informationrecording/reproducing apparatus and an optical informationrecording/reproducing method which determine in a short time voltagesapplied to the respective regions of the pattern electrode of the liquidcrystal optical element for correcting astigmatism so that the qualityof a reproduced signal is best improved. This is because astigmatism ofan arbitrary direction and amount is corrected through independentlycorrecting the astigmatism between the 0° and 90° directions and theastigmatism between the 45° and 135° directions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of an optical head of anoptical information recording/reproducing apparatus in exemplaryembodiments of the present invention;

FIG. 2 is a plan view of a pattern electrode of a liquid crystal opticalelement for correcting astigmatism in a first exemplary embodiment ofthe present invention;

FIGS. 3A to 3D are diagrams showing wave aberration when astigmatism iscorrected by liquid crystal optical elements according to first andsecond exemplary embodiments of the present invention;

FIGS. 4A to 4C are sectional views of a liquid crystal optical elementaccording to the exemplary embodiments of the present invention;

FIG. 5 is a diagram showing the relationship between the voltagesapplied to the electrode of the liquid crystal optical element and thephase of light transmitted through the liquid crystal optical elementaccording to exemplary embodiments of the present invention;

FIG. 6 is a diagram showing the relationship between regions of thepattern electrode of the liquid crystal optical element for correctingastigmatism and applied voltages in the first exemplary embodiment;

FIGS. 7A to 7E are diagrams showing waveforms of the voltagesrespectively applied to the regions of the pattern electrode of theliquid crystal optical element for correcting astigmatism in the firstexemplary embodiment of the present invention;

FIG. 8 is a diagram showing a waveform of a voltage applied to an entiresurface electrode of the liquid crystal optical element for correctingastigmatism in first to fourth exemplary embodiments of the presentinvention;

FIG. 9 is a plan view of a pattern electrode of the liquid crystaloptical element for correcting astigmatism in the second exemplaryembodiment of the present invention;

FIG. 10 is a diagram showing the relationship between regions of thepattern electrode of the liquid crystal optical element for correctingastigmatism and applied voltages in the second exemplary embodiment;

FIGS. 11A to 11E are diagrams showing waveforms of the voltages appliedto the respective regions of the pattern electrode of the liquid crystaloptical element for correcting astigmatism in the second exemplaryembodiment of the present invention;

FIG. 12 is a plan view of a pattern electrode of the liquid crystaloptical element for correcting astigmatism in the third exemplaryembodiment of the present invention;

FIG. 13 is a diagram showing relationship between regions of the patternelectrode of the liquid crystal optical element for correctingastigmatism and applied voltages in the third exemplary embodiment;

FIG. 14 is a plan view of a pattern electrode of the liquid crystaloptical element for correcting astigmatism in the fourth exemplaryembodiment of the present invention;

FIG. 15 is a diagram showing relationship between regions of the patternelectrode of the liquid crystal optical element for correctingastigmatism and applied voltages in the fourth exemplary embodiment;

FIG. 16 is a plan view of a pattern electrode of a liquid crystaloptical element for correcting coma aberration in a fifth exemplaryembodiment of the present invention;

FIGS. 17A to 17D are diagrams showing wave aberration when comaaberration is corrected by the liquid crystal optical element in thefifth exemplary embodiment of the present invention;

FIG. 18 is a diagram showing relationship between regions of the patternelectrode of the liquid crystal optical element for correcting comaaberration and applied voltages in the fifth exemplary embodiment;

FIG. 19 is a plan view of a pattern electrode of a liquid crystaloptical element for correcting spherical aberration in a sixth exemplaryembodiment of the present invention;

FIGS. 20A to 20D are diagrams showing wave aberration when sphericalaberration is corrected by the liquid crystal optical element in thesixth exemplary embodiment of the present invention;

FIG. 21 is a diagram showing relationship between regions of the patternelectrode of the liquid crystal optical element for correcting sphericalaberration and applied voltages in the sixth exemplary embodiment;

FIGS. 22A to 22C are diagrams showing waveforms of the voltages appliedto the respective regions of the pattern electrode of the liquid crystaloptical element for correcting coma aberration or spherical aberrationin the fifth and sixth exemplary embodiments of the present invention;

FIG. 23 is a diagram showing a configuration of the optical informationrecording/reproducing apparatus according to the exemplary embodimentsof the present invention;

FIGS. 24A and 24B are diagrams showing an example of measurement of aquality evaluation index of a reproduced signal by an opticalinformation recording/reproducing method of the present invention;

FIGS. 25A and 25B are diagrams showing an example of measurement of thequality evaluation index of the reproduced signal by the opticalinformation recording/reproducing method of the present invention;

FIG. 26 is a diagram showing an example of measurement of the qualityevaluation index of the reproduced signal by the optical informationrecording/reproducing method of the present invention;

FIG. 27 is a diagram showing an example of measurement of the qualityevaluation index of the reproduced signal by the optical informationrecording/reproducing method of the present invention;

FIG. 28 is a diagram showing a configuration of a conventional opticalhead provided with a liquid crystal optical element for correctingastigmatism;

FIG. 29 is a plan view of a pattern electrode of the liquid crystaloptical element for correcting astigmatism in the conventional opticalhead; and

FIG. 30 is a diagram showing relationship between regions of the patternelectrode of the liquid crystal optical element for correctingastigmatism and applied voltages in the conventional optical head.

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter, exemplary embodiments of the present invention will bedescribed with reference to the drawings.

FIG. 23 shows the configuration of an optical informationrecording/reproducing apparatus according to exemplary embodiments ofthe present invention. The optical information recording/reproducingapparatus includes: a recording signal generation circuit 11; asemiconductor laser drive circuit 12; a preamplifier 13; a reproducedsignal generation circuit 14; a liquid crystal optical element drivecircuit 15; an error signal generation circuit 16; an objective lensdrive circuit 17; and an optical head 50.

The recording signal generation circuit 11 generates a recording signalfor driving the optical head 50 on the basis of recording data inputtedexternally and outputs the recording signal to the semiconductor laserdrive circuit 12. The semiconductor laser drive circuit 12 drives asemiconductor laser 1 included within the optical head 50 on the basisof the recording signal received from the recording signal generationcircuit 11. This achieves signal recording on a disk 7. Details of theoptical head 50 will be described later. On the other hand, thepreamplifier 13 converts a current signal received from a photo-detector10 included within the optical head 50 into a voltage signal. Thereproduced signal generation circuit 14 generates a reproduced signalfor driving a liquid crystal optical element 4 included within theoptical head 50 on the basis of the voltage signal received from thepreamplifier 13, and externally outputs reproduced data. This achievessignal reproduction from the disk 7. The liquid crystal optical elementdrive circuit 15 drives the liquid crystal optical element 4 included inthe optical head 50 on the basis of the reproduced signal inputted fromthe reproduced signal generation circuit 14, so that the qualityevaluation index of the reproduced signal is best improved. As a result,various aberration correction operations are performed. Moreover, theerror signal generation circuit 16 generates a focus error signal and atruck error signal for driving an objective lens 6 on the basis of thevoltage signal received from the preamplifier 13. The objective lensdrive circuit 17 drives the objective lens 6 by an actuator (not shown)on the basis of the focus error signal and truck error signal inputtedfrom the error signal generation circuit 16 so that the focus errorsignal and the truck error signal are reduced to zero. This achievesfocus servo and truck servo operations. In addition, the opticalinformation recording/reproducing apparatus includes: a spindle controlcircuit for rotating the disk 7; a positioner control circuit for movingthe entire optical head 50 with respect to the disk 7 and so on (notshown).

Described in this exemplary embodiment is a recording/reproducing devicewhich performs recording and reproduction on the disk 7. Besides,possible exemplary embodiments of the optical informationrecording/reproducing apparatus of the present invention include areproduction-dedicated device that performs only reproduction on thedisk 7. In this case, the semiconductor laser 1 included within theoptical head 50 is not driven by the semiconductor laser drive circuit12 on the basis of the recording signal, but kept driven with a constantoutput.

FIG. 1 shows the configuration of the optical head 50 in the exemplaryembodiments of the present invention. The optical head 50 includes: thesemiconductor laser 1; a collimator lens 2; a polarizing beam splitter3; the liquid crystal optical element 4; a quarter-wave plate 5; theobjective lens 6; a cylindrical lens 8; a concave lens 9; and thephoto-detector 10. Light emitted from the semiconductor laser 1 which isa light source is collimated by the collimator lens 2, and is madeincident as the P-polarized light on the polarizing beam splitter 3serving as the light splitting means. The polarizing beam splitter 3transmits the incident light therethrough almost completely to output tothe liquid crystal optical element 4. The liquid crystal optical element4, which is provided for the aberration correction of the transmittedlight, transmits the incident light. The light transmitted through theliquid crystal optical element 4 is transmitted through the quarter-waveplate 5 and converted from linear polarized light into circularpolarized light. The circular polarized light is focused onto the disk 7serving as an optical recording medium by the objective lens 6.

The reflection light reflected by the disk 7 is transmitted backwardthrough the objective lens 6, and then made incident on the quarter-waveplate 5. The quarter-wave plate 5 converts the transmitted light fromthe circular polarized light into linear polarized light whosepolarization direction is orthogonal to that on the forward path, andoutputs the linear polarized light. The light converted into linearpolarized light is transmitted backward through the liquid crystaloptical element 4, and made incident as S polarized light on thepolarizing beam splitter 3. The light almost completely reflected by thepolarizing beam splitter 3 toward the cylindrical lens 8 is transmittedthrough the cylindrical lens 8 and the concave lens 9, and then receivedby the photo-detector 10. The photo-detector 10 is positioned in themiddle between two focal lines formed by the cylindrical lens 8 and theconcave lens 9. The photo-detector 10 has light receiving partsquarterly divided by a parting line parallel to the radial direction ofthe disk 7 and a parting line parallel to the tangential directionthereof. On the basis of outputs from the respective light receivingpart, a focus error signal is obtained by an astigmatism method, a truckerror signal is obtained by a phase difference method or a push-pullmethod, and a reproduced signal is obtained.

FIGS. 4A to 4C show sectional views of the liquid crystal opticalelement 4. The liquid crystal optical element 4 is structured to haveliquid crystal polymer 25 sandwiched between two substrates 23 a and 23b. An electrode 24 a is formed on the surface of the substrate 23 a onthe side of the liquid crystal polymer 25, and an electrode 24 b isformed on the surface of the substrate 23 b on the side of the liquidcrystal polymer 25. For the case that the liquid crystal optical element4 provides astigmatism correction, the electrode 24 a is a patternelectrode 18 having a pattern for correcting astigmatism, and theelectrode 24 b is an entire surface electrode. Arrows in the figuresshow the longitudinal direction of the liquid crystal polymer 25. Theliquid crystal polymer 25 has uniaxial refractive index anisotropy suchthat the direction of the optical axis is in the longitudinal direction.Therefore, the refractive index for the light component polarized in thedirection parallel to the longitudinal direction (abnormal lightcomponent) is larger than the reflective index for light componentpolarized in the direction vertical to the longitudinal direction(normal light component).

Light on the forward path within the optical head 50 is made incident onthe liquid crystal optical element 4 as linear polarized light whosepolarization direction is parallel to the plane of the figures. In FIG.4A, the angle formed by the longitudinal direction of the liquid crystalpolymer 25 and the polarization direction of the light incident on theliquid crystal optical element 4 is 90°. Therefore, the light incidenton the liquid crystal optical element 4 has only a normal lightcomponent; thus, the refractive index for this light is a smallrefractive index for the normal light component. In FIG. 4B, the angleformed by the longitudinal direction of the liquid crystal polymer 25and the polarization direction of the light incident on the liquidcrystal optical element 4 is in the middle between 90° and 0°.Therefore, the light incident on the liquid crystal optical element 4has both a normal light component and an abnormal light component; thus,the refractive index for this light is a refractive index in the middlebetween a small refractive index for the normal light component and alarge refractive index for the abnormal light component. In FIG. 4C, theangle formed by the longitudinal direction of the liquid crystal polymer25 and the polarization direction of the light incident on the liquidcrystal optical element 4 is 0°. Therefore, the light incident on theliquid crystal optical element 4 has only an abnormal light component;thus, the refractive index for this light is a large refractive indexfor the abnormal light component.

FIG. 5 shows relationship between the voltage applied between theelectrodes of the liquid crystal optical element 4 and the phase oflight transmitted through the liquid crystal optical element 4. As thevoltage applied between the electrodes 24 a and 24 b of the liquidcrystal optical element 4 is increased, the angle formed by thelongitudinal direction of the liquid crystal polymer 25 and thepolarization direction of the light incident on the liquid crystaloptical element 4 is increased from the state of FIG. 4C to the state ofFIG. 4A. In this case, the refractive index for the light incident onthe liquid crystal optical element 4 is reduced, and thus the phase ofthe light transmitted through the liquid crystal optical element 4 isadvanced. As shown in FIG. 5, when the voltage applied between theelectrodes 24 a and 24 b of the liquid crystal optical element 4 iswithin the range of V0±ΔV and the phase of the light transmitted throughthe liquid crystal optical element 4 is within the range of φ0±Δφ, therelationship between the voltage applied between the electrodes 24 a and24 b of the liquid crystal optical element 4 and the phase of the lighttransmitted through the liquid crystal optical element 4 issubstantially linear. Usually, the voltage V0 is approximately 2.5 voltsand the voltage ΔV is approximately 0.5 volts.

Next, a description is given of astigmatism. When X and Y axes aredefined in the radial direction and tangential direction of an opticalrecording medium, respectively, wave aberration caused by astigmatism ofarbitrary direction and magnitude can be expressed by a quadraticfunction of X and Y. A general form of the quadratic function of X and Yis given as AX²+2BXY+CY² (where A, B, and C are constant numbers).Therefore, correction of the astigmatism of arbitrary direction andmagnitude can be achieved if the wave aberration expressed by thisformula can be corrected. This formula is modified into(A+C)(X²+Y²)/2+(A−C)(X²⁻Y²)/2+2BXY, where (X²+Y²) denotes waveaberration caused by defocus aberration, (X²⁻Y²) denotes wave aberrationcaused by astigmatism between the 0° direction and the 90° direction,and 2XY denotes wave aberration caused by astigmatism between the 45°direction and the 135° direction.

The defocus aberration is automatically corrected by focus servo, andtherefor the correction of the astigmatism of arbitrary direction andmagnitude is achieved if the astigmatism between the 0° direction andthe 90° direction and the astigmatism between the 45° direction and the135° direction are simultaneously corrected by the liquid crystaloptical element. Notedly, (A−C)/2, which is the coefficient of (X²−Y²)and B, which is the coefficient of 2XY, are independent from each other.Therefore, a change in the magnitude of one of the astigmatism betweenthe 0° direction and the 90° direction and the astigmatism between the45° direction and the 135° direction does not result in a change in themagnitude of the other astigmatism.

[First Exemplary Embodiment]

In a first exemplary embodiment of the present invention, the liquidcrystal optical element 4 for correcting astigmatism is provided with: apattern electrode 18 a having a pattern for correcting astigmatism asthe electrode 24 a or 24 b formed on the surface of one substrate on theliquid crystal polymer side; and an entire surface electrode formed onthe surface of the other substrate on the liquid crystal polymer side.FIG. 2 is a plan view of the pattern electrode 18 a of the liquidcrystal optical element 4 for correcting astigmatism in the firstexemplary embodiment. In the figure, X and Y axes respectivelycorrespond to the radial and tangential directions of the disk 7. Thepattern electrode 18 a is, as shown in FIG. 2, divided into nineregions. Specifically, the pattern electrode 18 a is divided into: aregion 19 a formed in the shape of circle with an optical axis servingas a center; and regions 19 b to 19 i which are positioned outside theregion 19 a and divided by four straight lines passing through theoptical axis in units of 45 degrees in accordance with the angle aroundthe optical axis. The dotted line in the figure indicates the effectivediameter of the objective lens 6.

FIGS. 3A to 3D show wave aberration when astigmatism is corrected withthe liquid crystal optical element 4. FIGS. 3A to 3D show the waveaberration on the cross section in the X-axis or Y-axis directionpassing through the optical axis, describing the astigmatism between the0° and 90° directions. FIGS. 3A to 3D also shows wave aberration on thecross section in the 45° direction with respect to the X-axis directionpassing through the optical axis or in the 45° direction with respect tothe Y-axis direction, for the astigmatism between the 45° direction andthe 135° direction. Solid lines of FIGS. 3A and 3B show the waveaberration caused by astigmatism to be corrected. The liquid crystaloptical element drive circuit 15 corrects the wave aberration caused byastigmatism by controlling voltages applied to the pattern electrode 18a of the liquid crystal optical element 4. Dotted lines of FIGS. 3A and3B show the wave aberration caused by the liquid crystal optical element4 that corrects the astigmatism. Solid lines of FIGS. 3C and 3D showremaining wave aberration when the astigmatism is corrected by theliquid crystal optical element 4.

When the coefficient of X²−Y², which indicates the wave aberrationcaused by the astigmatism to be corrected, is negative, as shown by thesolid line of FIG. 3A in which the horizontal axis is defined as theX-axis, the wave aberration on the cross section in the X-axis directionpassing through the optical axis changes in the form of a quadraticfunction, i.e., to negative values, to 0, and to negative values, as itgoes from the negative side to the positive side of the X-axis.Moreover, as shown by the solid line of FIG. 3B in which the horizontalaxis is defined as the Y-axis, the wave aberration on the cross sectionin the Y-axis direction passing through the optical axis changes in theform of a quadratic function, i.e., to positive values, to 0, and topositive values, as it goes from the negative side to positive side ofthe Y-axis.

When the astigmatism is corrected by the liquid crystal optical element4, as shown by the dotted line in FIG. 3A, the wave aberration caused bythe liquid crystal optical element 4 on the cross section in the X-axisdirection passing through the optical axis changes in a staircasepattern, i.e., to a positive value, to 0, and to the positive value asit goes from the negative side to positive side of the X-axis. Moreover,as shown by the dotted line in FIG. 3B, the wave aberration caused bythe liquid crystal optical element 4 on the cross section in the Y-axisdirection passing through the optical axis changes in a stair casepattern, i.e., to a negative value, to 0, and to the negative value fromthe negative side toward positive side of the Y-axis. When the amount ofthe astigmatism corrected by the liquid crystal optical element 4 isadjusted optimally, the RMS (root-mean-square) of the remaining waveaberration after the astigmatism correction is minimized. FIG. 3C showsthe remaining wave aberration on the cross section in the X-axisdirection passing through the optical axis in this condition, that is,the superposition of the wave aberration indicated by the solid line andthe wave aberration indicated by the dotted line in FIG. 3A. As isunderstood from FIG. 3C, the absolute value of the remaining waveaberration on the cross section in the X-axis direction passing throughthe optical axis is reduced to around zero. Moreover, FIG. 3D shows theremaining wave aberration on the cross section in the Y-axis directionpassing through the optical axis in this condition, that is, thesuperposition of the wave aberration indicated by the solid line and thewave aberration indicated by the dotted line in FIG. 3B. As isunderstood by FIG. 3D, the absolute value of the remaining waveaberration on the cross section in the Y-axis direction passing throughthe optical axis is reduced to around zero.

When the coefficient of X²−Y², which indicates the wave aberrationcaused by the astigmatism to be corrected, is positive, as shown by thesolid line of FIG. 3B in which the horizontal axis is defined as theX-axis, the wave aberration on the cross section in the X-axis directionpassing through the optical axis changes in the form of a quadraticfunction, i.e., to positive values, to 0, and to positive values as itgoes from the negative side toward positive side of the X-axis.Moreover, as shown by the solid line of FIG. 3A in which the horizontalaxis is defined as the Y-axis, the wave aberration on the cross sectionin the Y-axis direction passing through the optical axis changes in theform of a quadratic function, i.e., to negative values, to 0, and tonegative values as it goes from the negative side toward positive sideof the Y-axis.

As shown by the dotted line in FIG. 3B, the wave aberration caused bythe liquid crystal optical element 4 on the cross section in the X-axisdirection passing through the optical axis changes in a stair casepattern, i.e., to a negative value, to 0, and to the negative value asit goes from the negative side toward positive side of the X-axis.Moreover, as shown by the dotted line in FIG. 3A, the wave aberrationcaused by the liquid crystal optical element 4 on the cross section inthe Y-axis direction passing through the optical axis changes in a staircase pattern, i.e., to a positive value, to 0, and to the positive valueas it goes from the negative side toward positive side of the Y-axis.When the amount of the astigmatism corrected by the liquid crystaloptical element 4 is adjusted optimally, the RMS (root-mean-square) ofthe remaining wave aberration after the stigmatism correction isminimized. FIG. 3D shows the remaining wave aberration on the crosssection in the X-axis direction passing through the optical axis in thiscondition, that is, the superposition of the wave aberration indicatedby the solid line and the wave aberration indicated by the dotted linein FIG. 3B. As is understood from FIG. 3D, the absolute value of theremaining wave aberration on the cross section in the X-axis directionpassing through the optical axis is reduced to around zero. Moreover,FIG. 3C shows the remaining wave aberration on the cross section in theY-axis direction passing through the optical axis in this condition,that is, a sum of the wave aberration indicated by the solid line andthe wave aberration indicated by the dotted line in FIG. 3A. As isunderstood from FIG. 3A, the absolute value of the remaining waveaberration on the cross section in the Y-axis direction passing throughthe optical axis is reduced to around zero.

Next, a method of controlling the liquid crystal optical element 4,which includes the pattern electrode 18 a to correct astigmatism, willbe described. Here, an effective voltage applied to the entire surfaceelectrode of the liquid crystal optical element 4 is defined as V4(hereinafter, expressed simply as voltage). Moreover, out of the regionsof the pattern electrode 18 a of the liquid crystal optical element 4,at least one region, for example, the region 19 a is defined as areference region, and the voltage applied to the reference region isdefined as V1. Note that V1+V4=V0. That is, the voltage V0 is appliedbetween the entire surface electrode and the reference region of thepattern electrode 18 a. Therefore, the phase of light transmittedthrough the liquid crystal optical element 4 in the reference region isφ0. When a region other than the reference region of the patternelectrode 18 a of the liquid crystal optical element 4 is fed with avoltage higher than V1 (that is, the absolute value thereof is higher),the phase of the light transmitted through the liquid crystal opticalelement 4 in this region is advanced with respect to φ0. That is, thelight transmitted through the liquid crystal optical element 4 in thisregion experience positive wave aberration with respect to the lighttransmitted through the liquid crystal optical element 4 in thereference region. On the other hand, when a region other than thereference region out of the pattern electrode 18 a of the liquid crystaloptical element 4 is fed with a voltage lower than V1 (the absolutevalue thereof is lower), the phase of the light transmitted through thisregion of the liquid crystal optical element 4 is delayed with respectto φ0. That is, the light transmitted through this region of the liquidcrystal optical element 4 experiences negative wave aberration withrespect to the light transmitted through the reference region of theliquid crystal optical element 4.

Therefore, the correction of the astigmatism between the 0° directionand the 90° direction is achieved by driving the pattern electrode 18 aof the liquid crystal optical element 4 in accordance with the followingtwo drive patterns, with the region 19 a defined as the referenceregion. The phase of transmission light transmitted through the regions19 b, 19 e, 19 f, and 19 i (referred to as a region group 191) isadvanced with respect to the phase of transmission light transmittedthrough the region 19 a, and the phase of transmission light transmittedthrough the regions 19 c, 19 d, 19 g, and 19 h (referred to as a regiongroup 192) is delayed with respect to the phase of the transmissionlight transmitted through the region 19 a. Alternatively, (2) the phaseof the transmission light transmitted through the region group 191 isdelayed with respect to the phase of the transmission light transmittedthrough the region 19 a, and the phase of the transmission lighttransmitted through the region group 192 is delayed with respect to thephase of the transmission light transmitted through the region 19 a.That is, a voltage (V1+Vα) is applied to the region group 191 and avoltage (V1−Vα) is applied to the region group 2, while the voltage V1is applied to the region 19 a.

Similarly, the correction of the astigmatism between the 45° directionand the 135° direction is achieved by driving the pattern electrode 18 aof the liquid crystal optical element 4 in accordance with the followingtwo drive patterns with the region 19 a defined as the reference region.The phase of transmission light transmitted through the regions 19 b, 19c, 19 f, and 19 g (referred to as a region group 193) is advanced withrespect to the phase of the transmission light transmitted through theregion 19 a, and the phase of transmission light transmitted through theregions 19 d, 19 e, 19 h, and 19 i (referred to as a region group 194)is delayed with respect to the phase of the transmission lighttransmitted through the region 19 a. Alternatively, (2) the phase of thetransmission light transmitted through the region group 193 is delayedwith respect to the phase of the transmission light transmitted throughthe region 19 a, and the phase of the transmission light transmittedthrough the region group 194 is delayed with respect to the phase of thetransmission light transmitted through the region 19 a. That is, avoltage (V1+Vβ) is applied to the region group 193 and a voltage (V1−Vβ)is applied to the region group 4, while the voltage V1 is applied to theregion 19 a.

FIG. 6 shows relationship between the respective regions of the patternelectrode 18 a of the liquid crystal optical element 4 for correctingastigmatism and the respective voltages applied to the respectiveregions. The voltage applied to the entire surface electrode of theliquid crystal optical element 4 is defined as the voltage V4. Moreover,the region 19 a, which is selected out of the regions of the patternelectrode 18 a of the liquid crystal optical element 4, is defined as areference region, and the voltage applied to the reference region 19 ais defined as the voltage V1. Note that V1+V4=V0. The voltage applied tothe regions 19 b and 19 f is V1+Vα+Vβ, the voltage applied to theregions 19 c and 19 g is V1−Vα+Vβ, the voltage applied to the regions 19d and 19 h is V1−Vα−Vβ, and the voltage applied to the regions 19 e and19 i is V1+Vα−Vβ.

Here, assuming that the voltage Vβ=0 for simplicity, for Vα>0, the lighttransmitted through the region group 191 of the liquid crystal opticalelement 4 is advanced in phase with respect to the light transmittedthrough the reference region 19 a of the liquid crystal optical element4, and the light transmitted through the region group 192 of the liquidcrystal optical element 4 is delayed in phase. Moreover, for Vα<0, thelight transmitted through the region group 191 of the liquid crystaloptical element 4 is delayed in phase with respect to the lighttransmitted through the reference region 19 a of the liquid crystaloptical element 4, and the light transmitted through the liquid crystaloptical element 4 in the region group 192 is advanced in phase. That is,the change in the voltage Vα causes a change in the correction amount ofastigmatism between the 0° direction and the 90° direction. The signs ofcorrectable astigmatism for Vα>0 and Vα<0 are opposite to each other.The absolute value of the amount of correctable astigmatism is increasedwith the increase in the absolute value of the voltage Vα.

Assuming that the voltage Vα=0, on the other hand, for Vβ>0, the lighttransmitted through the region group 193 of the liquid crystal opticalelement 4 is advanced in phase with respect to the light transmittedthrough the reference region 19 a of the liquid crystal optical element4, and the light transmitted through the region group 194 of the liquidcrystal optical element 4 is delayed in phase. Moreover, for Vβ<0, thelight transmitted through the region group 193 of the liquid crystaloptical element 4 is delayed in phase with respect to the lighttransmitted through the reference region 19 a of the liquid crystaloptical element, and the light transmitted through the region group 194of the liquid crystal optical element 4 is advanced in phase. That is,the change in the voltage Vβ causes the change in the correction amountof astigmatism between the 45° direction and the 135° direction. Thesigns of correctable astigmatism for Vβ>0 and Vβ<0 are opposite to eachother. The absolute value of the amount of correctable astigmatism isincreased with the increase in the absolute value of the voltage Vβ.

Although the above is described with an assumption that Vβ=0 or Vα=0 forsimplicity, the voltage Vα and the voltage Vβ are actually changeablewithin a range in which the sum of the absolute values of the voltagesVα and Vβ does not exceed a voltage ΔV. That is, the voltage Vα and thevoltage Vβ are set so that it holds |Vα|+|Vβ|≦ΔV.

FIGS. 7A to 7E show waveforms of the voltages applied to the respectiveregions of the pattern electrode 18 a of the liquid crystal opticalelement 4 for correcting astigmatism. In FIGS. 7A to 7E, the horizontalaxis denotes the time, and the vertical axis denotes the voltage.Long-term application of a dc voltage to the electrode of the liquidcrystal optical element causes destruction of the liquid crystalpolymer. Thus, in practice, an alternating voltage is applied. As shownin FIGS. 7A to 7E, in-phase rectangular wave voltages of a frequency ofapproximately one kilohertz are applied to the respective regions of thepattern electrode 18 a. As shown in FIG. 7A, the region 19 a of thepattern electrode 18 a of the liquid crystal optical element 4 isapplied with a rectangular wave voltage with an amplitude of ±V1, andthe effective voltage thereof is V1. As shown in FIG. 7B, the region 19b and the region 19 f is applied with a rectangular wave voltage with anamplitude of ±(V1+Vα+Vβ), and the effective voltage thereof is(V1+Vα+Vβ). As shown in FIG. 7C, the region 19 c and the region 19 g isapplied with a rectangular wave voltage with an amplitude of±(V1−Vα+Vβ), and the effective voltage thereof is (V1−Vα+Vβ). As shownin FIG. 7D, the region 19 d and the region 19 h is applied with arectangular wave with an amplitude of ±(V1−Vα−Vβ), and the effectivevoltage thereof is (V1−Vα−Vβ). As shown in FIG. 7E, the region 19 e andthe region 19 i is applied with a rectangular wave voltage with anamplitude of ±(V1+Vα−Vβ), and the effective voltage thereof is(V1+Vα−Vβ).

FIG. 8 shows a waveform of the voltage applied to the entire surfaceelectrode of the liquid crystal optical element 4 for correctingastigmatism. In FIG. 8, the horizontal axis denotes the time, and thevertical axis denotes the voltage. As shown in FIG. 8, the entiresurface electrode of the liquid crystal optical element 4 is appliedwith a rectangular wave voltage having a frequency of approximately 1kHz with an amplitude of ±V4, and the effective voltage thereof is V4.The waveform of the voltage applied to the entire surface electrode isopposite in phase to the waveforms of the voltages applied to theregions 19 a to 19 i of the pattern electrode 18 a of the liquid crystaloptical element 4. Therefore, an effective voltage of V0±Vα±Vβ isapplied between the entire surface electrode and the pattern electrode18 a, and the phase varies with respect to φ0 as center.

As described above, a change in the voltage Vα causes a change in thecorrection amount of astigmatism between the 0° and 90° directions,while a change in the voltage Vβ causes a change in the correctionamount of astigmatism between the 45° and 135° directions. Therefore,determining the voltage Vα with the voltage Vβ fixed so that the qualityevaluation index of the reproduced signal is best improved allowsobtaining the optimum correction amount such that remaining waveaberration caused by the astigmatism between the 0° and 90° directionsis minimized. Moreover, determining the voltage Vβ with the voltage Vαfixed so that the quality evaluation index of the reproduced signal isbest improved allows obtaining an optimum correction amount such thatremaining wave aberration caused by the astigmatism between the 45° and135° directions is minimized.

A change in the amount of one of the astigmatism between the 0° and 90°directions and the astigmatism between the 45° and 135° directions doesnot result in a change in the amount of the other. Therefore, thevoltage Vα such that the quality evaluation index of the reproducedsignal is best improved (optimum voltage Vα0) does not depend on thevoltage Vβ, and the voltage Vβ such that the quality evaluation index ofthe reproduced signal is best improved (optimum voltage Vβ0) does notdepend on the voltage Vα. Therefore, whichever of the step fordetermining the optimum voltage Vα0 with the Vβ fixed and the step fordetermining the optimum voltage Vβ0 with the Vα fixed is carried outfirst, the same combination of the determined optimum voltages Vα0 andVβ0 is obtained.

After the determination of the optimum voltages Vα0 and Vβ0, theaforementioned drive voltages are applied to the respective regions ofthe pattern electrode of the liquid crystal optical element on the basisof the combination determined. This allows simultaneously minimizing theremaining wave aberration caused by the astigmatism between the 0° and90° directions and the remaining wave aberration caused by theastigmatism between the 45° and 135° directions. That is, astigmatism ofthe arbitrary direction and amount can be corrected. At this case, thequality of the reproduced signal from the optical recording medium isbest improved. Moreover, in determining the combination of the optimumvoltages Vα0 and Vβ0, the quality evaluation index of the reproducedsignal is not measured for all combinations of the voltages Vα and Vβ,the quality evaluation index of the reproduced signal is only measuredfor all the voltages Vα with the voltage Vβ fixed and for all thevoltages Vβ with the voltage Vα fixed. Thus, the combination of theoptimum voltages Vα0 and Vβ0 is determined in a short time.

[Second Exemplary Embodiment]

In a second exemplary embodiment of the present invention, the liquidcrystal optical element 4 for correcting astigmatism is structured tohave a liquid crystal polymer sandwiched between two substrates. Apattern electrode 18 b having a pattern for correcting astigmatism isformed on the surface of one of the substrates on the liquid crystalpolymer side, and an entire surface electrode is formed on the surfaceof the other substrate on the liquid crystal polymer side. FIG. 9 is aplan view of the pattern electrode 18 b of the liquid crystal opticalelement 4 for correcting astigmatism in the second exemplary embodiment.In the figure, X and Y axes respectively correspond to the radial andtangential directions of the disk 7. As shown in FIG. 9, the patternelectrode 18 b is divided into nine regions. Specifically, the patternelectrode 18 b is divided into: a region 19 a formed in the circularshape with the optical axis as center; and regions 19 j to 19 q whichare positioned outside the region 19 a and divided by four straightlines passing through the optical axis in units of 45 degrees inaccordance with the angle around the optical axis. The dotted line inthe figure corresponds to the effective diameter of the objective lens6.

Wave aberration when astigmatism is corrected by the liquid crystaloptical element 4 in the second exemplary embodiment is same as thatshown in FIGS. 3A to 3D. Moreover, sectional views of the liquid crystaloptical element 4 in the second exemplary embodiment are the same asthose shown in FIG. 4. Further, the relationship between voltagesapplied to the electrode of the liquid crystal optical element 4 and thephase of light transmitted through the liquid crystal optical element 4in the second exemplary embodiment is same as that shown in FIG. 5.

FIG. 10 shows relationship between the regions of the pattern electrode18 b of the liquid crystal optical element 4 for correcting astigmatismand the voltages applied to the respective regions in the secondexemplary embodiment. Here, the voltage applied to the entire surfaceelectrode of the liquid crystal optical element 4 is defined as thevoltage V4. Moreover, out of the regions of the pattern electrode 18 bof the liquid crystal optical element 4, the region 19 a is defined as areference region, and the voltage applied to the reference region isdefined as the voltage V1. Note that V1+V4=V0. That is, the voltage V0is applied between the entire surface electrode and the reference region19 a of the pattern electrode 18 b. Therefore, the phase of lighttransmitted through the reference region 19 a of the liquid crystaloptical element 4 is φ0. The voltage applied to the regions 19 j and 19n is V1+Vα, the voltage applied to the regions 19 k and 19 o is V1+Vβ,the voltage applied to the regions 19 l and 19 p is V1−Vα, and thevoltage applied to the regions 19 m and 19 q is V1−Vβ.

Here, assuming that the voltage Vβ=0 for simplicity, for Vα>0, lighttransmitted through the regions 19 j and 19 n of the liquid crystaloptical element 4 is advanced in phase with respect to the lighttransmitted through the reference region 19 a of the liquid crystaloptical element 4, and light transmitted through the regions 19 l and 19p of the liquid crystal optical element 4 is delayed in phase, and thephase of light transmitted through the regions 19 k, 19 m, 19 o, and 19q of the liquid crystal optical element 4 is same. Moreover, for Vα<0,the light transmitted through the regions 19 j and 19 n of the liquidcrystal optical element 4 is delayed in phase with respect to the lighttransmitted through the reference region 19 a of the liquid crystaloptical element 4, and the light transmitted through the regions 19 land 19 p of the liquid crystal optical element 4 is advanced in phase,and the light transmitted through the regions 19 k, 19 m, 19 o, and 19 qof the liquid crystal optical element 4 is equal in phase. Therefore, achange in the voltage Vα causes a change in the correction amount ofastigmatism between the 0° and 90° directions. The signs of correctableastigmatism for Vα>0 and Vα<0 are opposite to each other. The absolutevalue of the amount of correctable astigmatism is increased with anincrease in the absolute value of the voltage Vα.

Assuming that the voltage Vα=0, on the other hand, for Vβ>0, lighttransmitted through the regions 19 k and 19 o of the liquid crystaloptical element 4 is advanced in phase with respect to the lighttransmitted through the reference region 19 a of the liquid crystaloptical element 4, light transmitted through the regions 19 m and 19 qof the liquid crystal optical element 4 is delayed in phase, and lighttransmitted through the regions 19 j, 19 l, 19 n, and 19 p of the liquidcrystal optical element 4 is equal in phase. Moreover, for Vβ<0, thelight transmitted through the regions 19 k and 19 o of the liquidcrystal optical element 4 is delayed in phase with respect to the lighttransmitted through the reference region 19 a of the liquid crystaloptical element 4, the light transmitted through the regions 19 m and 19q of the liquid crystal optical element 4 is advanced in phase, andlight transmitted through the regions 19 j, 19 l, 19 n, and 19 p of theliquid crystal optical element 4 is equal in phase. That is, a change inthe voltage Vβ causes a change in the correction amount of astigmatismbetween the 45° and 135° directions. The signs of correctableastigmatism for Vβ>0 and Vβ<0 are opposite to each other. The absolutevalue of the amount of correctable astigmatism is increased with theincrease in the absolute value of the voltage Vβ.

Although the above is described with an assumption that Vβ=0 or Vα=0 forsimplicity, the voltages Vα and Vβ may be actually changed within arange such that the sum of the absolute values of the voltages Vα and Vβdoes not exceed the voltage ΔV. That is, the voltage Vα and the voltageVβ are set so that it holds |Vα|+|Vβ|≦ΔV.

FIGS. 11A to 11E show waveforms of the voltages applied to therespective regions of the pattern electrode 18 b of the liquid crystaloptical element 4 for correcting astigmatism in the second exemplaryembodiment. In FIGS. 11A to 11E, the horizontal axis denotes the time,and the vertical axis denotes the voltage. Long-term application of a dcvoltage to the electrode of the liquid crystal optical element causesdestruction of the liquid crystal polymer. Therefore, an ac voltage isactually applied. As shown in FIGS. 11A to 11E, in-phase rectangularwave voltages having a frequency of approximately one kilohertz areapplied to the respective regions of the pattern electrode 18 b. Asshown in FIG. 11A, the region 19 a of the pattern electrode 18 b of theliquid crystal optical element 4 is applied with a rectangular wavevoltage with an amplitude of ±V1, and the effective voltage thereof isV1. As shown in FIG. 11B, the regions 19 j and 19 n are applied with arectangular wave voltage with an amplitude of ±(V1+Vα), and theeffective voltage thereof is (V1+Vα). As shown in FIG. 11C, the regions19 k and 19 o are applied with a rectangular wave voltage with anamplitude of ±(V1+Vβ), and the effective voltage thereof is (V1+Vβ). Asshown in FIG. 11D, the regions 19 l and 19 p are applied with arectangular wave voltage with an amplitude of ±(V1−Vα), and theeffective voltage thereof is (V1−Vα). As shown in FIG. 11E, the regions19 m and 19 q are applied with a rectangular wave voltage with anamplitude of ±(V1−Vβ), and the effective voltage thereof is (V1−Vβ). Thewaveform of the voltage applied to the entire surface electrode of theliquid crystal optical element 4 is same as that shown in FIG. 8.

[Third Exemplary Embodiment]

In a third exemplary embodiment of the present invention, the liquidcrystal optical element 4 for correcting astigmatism is structured tohave a liquid crystal polymer sandwiched between two substrates. On thesurface of one of the substrates on the liquid crystal polymer side, apattern electrode 18 c having a pattern for correcting astigmatism isformed, and on the surface of the other substrate on the liquid crystalpolymer side, an entire surface electrode is formed. FIG. 12 is a planview of the pattern electrode 18 c of the liquid crystal optical element4 for correcting astigmatism in the third exemplary embodiment. In thefigure, X and Y axes respectively correspond to the radial andtangential directions of the disk 7. The pattern electrode 18 c is, asshown in FIG. 12, divided into 17 regions. Specifically, the patternelectrode 18 c is divided into: a circular region 20 a with the opticalaxis as center; regions 20 b to 20 i which are positioned outside theregion 20 a and divided by four straight lines passing through theoptical axis in units of 45 degrees in accordance with the angle aroundthe optical axis; and regions 21 b to 21 i positioned outside theregions 20 b to 20 i and divided by the four straight lines passingthrough the optical axis in units of 45 degrees in accordance with theangle around the optical axis. The boundary between the regions 20 b to20 i and the regions 21 b to 21 i is a circle with the optical axis ascenter. A dotted line in the figure corresponds to the effectivediameter of the objective lens 6.

A description will be given of wave aberration for the case thatastigmatism is corrected by the liquid crystal optical element 4. Whenthe wave aberration caused by astigmatism to be corrected varies in theform of a quadratic function, i.e., to negative values, to zero, and tonegative values as it goes from the negative side to positive side ofthe horizontal axis, as shown by the solid line of FIG. 3A, the liquidcrystal optical element drive circuit 15 controls the voltages appliedto the pattern electrode 18 c of the liquid crystal optical element 4 tothereby generate wave aberration for correction. Specifically, when thecorrection is achieved by the liquid crystal optical element 4, the waveaberration caused by the liquid crystal optical element 4 varies in astair case pattern, i.e., to a second positive value, to a firstpositive value, to 0, to the first positive value, and to the secondpositive value as is goes from the negative side to the positive side ofthe horizontal axis. The second positive value is K times the firstpositive value, where K is a constant number larger than 1. Moreover,when the wave aberration caused by astigmatism to be corrected varies inthe form of a quadratic function, i.e., to positive values, to 0, and topositive values as it goes from the negative side to positive side ofthe horizontal axis, as shown by a solid line of FIG. 3B, the liquidcrystal optical element drive circuit 15 controls the voltages appliedto the pattern electrode 18 c of the liquid crystal optical element 4 tothereby generate wave aberration for correction. The wave aberrationcaused by the liquid crystal optical element 4 changes in a stair casepattern, i.e., to a second negative value, to a first negative value, to0, to the first negative value, and to the second negative value as itgoes from the negative side to the positive side of the horizontal axis.The second negative value is K times the first positive value, where Kis a constant number larger than 1. Optimally defining the amount of theastigmatism corrected by the liquid crystal optical element 4 allowsminimizing the RMS of the remaining wave aberration after theastigmatism correction. In the third exemplary embodiment, the RMS ofthe remaining wave aberration is made smaller, compared to the firstexemplary embodiment. The value of K is preferably approximately 2 to 4to make the RMS of the remaining wave aberration as small as possible.

Sectional views of the liquid crystal optical element 4 in this thirdexemplary embodiment are same as those shown in FIGS. 4A to 4C.Moreover, the relationship between the voltages applied to the electrodeof the liquid crystal optical element 4 and the phase of lighttransmitted through the liquid crystal optical element 4 is same asthose shown in FIG. 5.

FIG. 13 shows relationship between the regions of the pattern electrode18 c of the liquid crystal optical element 4 for correcting astigmatismand the voltages applied to the respective regions in the thirdexemplary embodiment. Here, the voltage applied to the entire surfaceelectrode of the liquid crystal optical element 4 is defined as thevoltage V4. Moreover, out of the regions of the pattern electrode 18 cof the liquid crystal optical element 4, the region 20 a is defined as areference region, and the voltage applied to the reference region isdefined as the voltage V1. Note that V1+V4=V0. That is, the voltage V0is applied between the entire surface electrode and the reference region20 a of the pattern electrode 18 c. Therefore, the phase of lighttransmitted through the reference region 20 a of the liquid crystaloptical element 4 is φ0. As shown in FIG. 13, the voltage applied to theregions 20 b and 20 f is V1+Vα+Vβ, the voltage applied to the regions 20c and 20 g is V1−Vα+Vβ, the voltage applied to the regions 20 d and 20 his V1−Vα−Vβ, and the voltage applied to the regions 20 e and 20 i isV1+Vα−Vβ. Further, the voltage applied to the regions 21 b and 21 f isV1+K·Vα+K·Vβ, the voltage applied to the regions 21 c and 21 g isV1−K·Vα+K·Vβ, the voltage applied to the regions 21 d and 21 h isV1−K·Vα−K·Vβ, and the voltage applied to the regions 21 e and 21 i isV1+K·Vα−K·Vβ.

Here, assuming that the voltage Vβ=0 for simplicity, for Vα>0, lighttransmitted through the regions 20 b, 20 e, 20 f, and 20 i of the liquidcrystal optical element 4 (hereinafter, referred to as the region group201) is advanced in phase with respect to the light transmitted throughthe reference region 20 a of the liquid crystal optical element 4, lighttransmitted through the regions 21 b, 21 e, 21 f and 21 i of the liquidcrystal optical element 4 (hereinafter referred to as the region group211) is further advanced in phase, light transmitted through the regions20 c, 20 d, 20 g, and 20 h of the liquid crystal optical element 4(hereinafter referred to as the region group 202) is delayed in phase,and light transmitted through the regions 21 c, 21 d, 21 g, and 21 h ofthe liquid crystal optical element 4 (hereinafter referred to as theregion group 212) is further delayed in phase. Moreover, for Vα<0, lighttransmitted through the region group 201 of the liquid crystal opticalelement 4 is delayed in phase with respect to the light transmittedthrough the reference region 20 a of liquid crystal optical element 4,light transmitted through the region group 211 of the liquid crystaloptical element 4 is further delayed in phase, light transmitted throughthe region group 202 of the liquid crystal optical element 4 is advancedin phase, and light transmitted through the region group 212 of theliquid crystal optical element 4 is further advanced in phase.Therefore, a change in the value of the voltage Vα causes a change inthe correction amount of astigmatism between the 0° and 90° directions.The signs of correctable astigmatism for Vα>0 and Vα<0 are opposite toeach other. The absolute value of the amount of correctable astigmatismis increased with the increase in the absolute value of the voltage Vα.

Assuming that the voltage Vα=0, on the other hand, for Vβ>0, lighttransmitted through the regions 20 b, 20 c, 20 f, and 20 g of the liquidcrystal optical element 4 (hereinafter referred to as the region group203) is advanced in phase with respect to the light transmitted throughthe reference region 20 a of the liquid crystal optical element 4, lighttransmitted through the regions 21 b, 21 c, 21 f, and 21 g of the liquidcrystal optical element 4 (hereinafter referred to as the region group213) is further advanced in phase, light transmitted through the regions20 d, 20 e, 20 h, and 20 i of the liquid crystal optical element 4(hereinafter referred to as a region group 204) is delayed in phase, andlight transmitted through the regions 21 d, 21 e, 21 h, and 21 i of theliquid crystal optical element 4 (hereinafter referred to as a regiongroup 214) is further delayed in phase. Moreover, for Vβ<0, lighttransmitted through the region group 203 of the liquid crystal opticalelement 4 is delayed in phase with respect to the light transmittedthrough the reference region 20 a of the liquid crystal optical element4, light transmitted through the region group 213 of the liquid crystaloptical element 4 is further delayed in phase, light transmitted throughthe region group 204 of the liquid crystal optical element 4 is advancedin phase, and light transmitted through the region group 214 of theliquid crystal optical element 4 is further advanced in phase.Therefore, a change in the value of the voltage Vβ causes a change inthe correction amount of astigmatism between the 45° and 135°directions. The signs of correctable astigmatism for Vβ>0 and Vβ<0 areopposite to each other. The absolute value of the amount of correctableastigmatism is increased with the increase in the absolute value of Vβ.

Although the above is described with an assumption that Vβ=0 or Vα=0 forsimplicity, the voltages Vα and Vβ may be actually changed within arange in which K times the sum of the absolute values of the voltages Vαand Vβ does not exceed the voltage ΔV. That is, the voltages Vα and Vβare set so that it holds |K·Vα|+|K·Vβ|≦ΔV.

A description will be given of waveforms of the voltages applied to therespective regions of the pattern electrode 18 c of the liquid crystaloptical element 4 for correcting astigmatism in the third exemplaryembodiment. Similarly the waveforms shown in FIGS. 7A to 7E and FIGS.11A to 11E, in-phase rectangular wave voltages having a frequency ofapproximately one kilohertz are applied to the respective regions. Theregion 20 a of the pattern electrode 18 c of the liquid crystal opticalelement 4 is applied with the rectangular wave voltage with an amplitudeof ±V1, and the effective voltage thereof is V1. The regions 20 b and 20f are applied with a rectangular wave voltage with an amplitude of±(V1+Vα+Vβ), and the effective voltage thereof is (V1+Vα+Vβ). Theregions 20 c and 20 g are applied with a rectangular wave voltage withan amplitude of ±(V1−Vα+Vβ), and the effective voltage thereof is(V1−Vα+Vβ). The regions 20 d and 20 h are applied with a rectangularwave voltage with an amplitude of ±(V1−Vα−Vβ), and the effective voltagethereof is (V1−Vα−Vβ). The regions 20 e and 20 i are applied with arectangular wave voltage with an amplitude of ±(V1+Vα−Vβ), and theeffective voltage thereof is (V1+Vα−Vβ). The regions 21 b and 21 f areapplied with a rectangular wave voltage with an amplitude of±(V1+K·Vα+K·Vβ), and the effective voltage thereof is (V1+K·Vα+K·Vβ).The regions 21 c and 21 g are applied with a rectangular wave voltagewith an amplitude of ±(V1−K·Vα+K·Vβ), and the effective voltage thereofis (V1−K·Vα+K·Vβ). The regions 21 d and 21 h are applied with arectangular wave voltage with an amplitude of ±(V1−K·Vα−K·Vβ), and theeffective voltage thereof is (V1−K·Vα−K·Vβ). The regions 21 e and 21 iare applied with a rectangular wave voltage with an amplitude of±(V1+K·Vα−K·Vβ), and the effective voltage thereof is (V1+K·Vα−K·Vβ).The waveform of the voltage applied to the entire surface electrode ofthe liquid crystal optical element 4 for correcting astigmatism is sameas that shown in FIG. 8.

[Fourth Exemplary Embodiment]

In a fourth exemplary embodiment of the present invention, the liquidcrystal optical element 4 for correcting astigmatism is structured tohave a liquid crystal polymer sandwiched between two substrates. On thesurface of one of the substrates on the liquid crystal polymer side, apattern electrode 18 d having a pattern for correcting astigmatism isformed, and on the surface of the other substrate on the liquid crystalpolymer side, an entire surface electrode is formed. FIG. 14 is a planview of the pattern electrode 18 d of the liquid crystal optical element4 for correcting astigmatism in the fourth exemplary embodiment. In thefigure, X and Y axes respectively correspond to the radial andtangential directions of the disk 7. The pattern electrode 18 d is, asshown in FIG. 14, divided into 17 regions. Specifically, the patternelectrode 18 d is divided into: a circular region 20 a with the opticalaxis as center; regions 20 j to 20 q which are positioned outside theregion 20 a and divided by four straight lines passing through theoptical axis in units of 45 degrees in accordance with the angle aroundthe optical axis; and regions 21 j to 21 q which are positioned outsidethe regions 20 j to 20 q and divided by the four straight lines passingthrough the optical axis in units of 45 degrees in accordance with theangle around the optical axis. The boundary between the regions 20 j to20 q and the regions 21 j to 21 q is a circle with the optical axis ascenter. The dotted line in the figure corresponds to the effectivediameter of the objective lens 6.

Wave aberration when astigmatism is corrected by the liquid crystaloptical element 4 in the fourth exemplary embodiment is same as thatshown in the third exemplary embodiment. Therefore, in the fourthexemplary embodiment, as compared to the second exemplary embodimentdescribed, the RMS of the remaining wave aberration can be made smaller.To make the RMS of the remaining wave aberration as small as possible,the value of K is preferably approximately 2 to 4.

Sectional views of the liquid crystal optical element 4 in this fourthexemplary embodiment are same as those shown in FIGS. 4A to 4C.Moreover, the relationship between the voltages applied to the electrodeof the liquid crystal optical element 4 and the phase of lighttransmitted through the liquid crystal optical element 4 are same asthose shown in FIG. 5.

FIG. 15 shows the relationship between the regions of the patternelectrode 18 d of the liquid crystal optical element 4 for correctingastigmatism and the voltages applied to the respective regions in thefourth exemplary embodiment. Here, the voltage applied to the entiresurface electrode of the liquid crystal optical element 4 is defined asthe voltage V4. Moreover, out of the regions of the pattern electrode 18d of the liquid crystal optical element 4, the region 20 a is defined asa reference region, and the voltage applied to the reference region isdefined as the voltage V1. Note that V1+V4=V0. That is, the voltage V0is applied between the entire surface electrode and the reference region20 a of the pattern electrode 18 d. Therefore, the phase of lighttransmitted through the reference region 20 a of the liquid crystaloptical element 4 is φ0. As shown in FIG. 15, the voltage applied to theregions 20 j and 20 n is V1+Vα, the voltage applied to the regions 20 kand 20 o is V1+Vβ, the voltage applied to the regions 20 l and 20 p isV1−Vα, and the voltage applied to the regions 20 m and 20 q is V1−Vβ.Further, the voltage applied to the regions 21 j and 21 n is V1+K·Vα,the voltage applied to the regions 21 k and 21 o is V1+K·Vβ, the voltageapplied to the regions 21 l and 21 p is V1−K·Vα, and the voltage appliedto the regions 21 m and 21 q is V1−K·Vβ.

Here, assuming that the voltage Vβ=0 for simplicity, for Vα>0, lighttransmitted through the regions 20 j and 20 n of the liquid crystaloptical element 4 is advanced in phase with respect to the lighttransmitted through the reference region 20 a of the liquid crystaloptical element 4, light transmitted through the regions 21 j and 21 nof the liquid crystal optical element 4 is further advanced in phase,light transmitted through the regions 20 l and 20 p of the liquidcrystal optical element 4 is delayed in phase, light transmitted throughthe regions 21 l and 21 p of the liquid crystal optical element 4 isfurther delayed in phase, and light transmitted through the regions 20k, 20 m, 20 o, 20 q, 21 k, 21 m, 21 o, and 21 q of the liquid crystaloptical element 4 is equal in phase. Moreover, for Vα<0, lighttransmitted through the regions 20 j and 20 n of the liquid crystaloptical element 4 is delayed in phase with respect to the lighttransmitted through the reference region 20 a of the liquid crystaloptical element 4, light transmitted through the regions 21 j and 21 nof the liquid crystal optical element 4 is further delayed in phase, thelight transmitted through the regions 20 l and 20 p of the liquidcrystal optical element 4 is advanced in phase, the light transmittedthrough the region 21 l and 21 p of the liquid crystal optical element 4is further advanced in phase, and light transmitted through the regions20 k, 20 m, 20 o, 20 q, 21 k, 21 m, 21 o, and 21 q of the liquid crystaloptical element 4 is equal in phase. Therefore, a change in the value ofthe voltage Vα causes a change in the correction amount of astigmatismbetween the 0° and 90° directions. The signs of correctable astigmatismfor Vα>0 and Vα<0 are opposite to each other. The absolute value of theamount of correctable astigmatism is increased with the increase in theabsolute value of the voltage Vα.

Assuming that the voltage Vα=0, on the other hand, for Vβ>0, lighttransmitted through the regions 20 k and 20 o of the liquid crystaloptical element 4 is advanced in phase with respect to the lighttransmitted through the reference region 20 a of the liquid crystaloptical element 4, light transmitted through the regions 21 k and 21 oof the liquid crystal optical element 4 is further advanced in phase,light transmitted through the regions 20 m and 20 q of the liquidcrystal optical element 4 is delayed in phase, light transmitted throughthe regions 21 m and 21 q of the liquid crystal optical element 4 isfurther delayed in phase, and light transmitted through the regions 20j, 20 l, 20 n, 20 p, 21 j, 21 l, 21 n, and 21 p of the liquid crystaloptical element 4 is equal in phase. Moreover, for Vβ<0, lighttransmitted through the regions 20 k and 20 o of the liquid crystaloptical element 4 is delayed in phase with respect to the lighttransmitted through the reference region 20 a of the liquid crystaloptical element 4, light transmitted through the regions 21 k and 21 oof the liquid crystal optical element 4 is further delayed in phase, thelight transmitted through the regions 20 m and 20 q of the liquidcrystal optical element 4 is advanced in phase, light transmittedthrough the regions 21 m and 21 q of the liquid crystal optical element4 is further advanced in phase, and light transmitted through theregions 20 j, 20 l, 20 n, 20 p, 21 j, 21 l, 21 n, and 21 p of the liquidcrystal optical element 4 is equal in phase. Therefore, a change in thevalue of the voltage Vβ causes a change in the correction amount ofastigmatism between the 45° and 135° directions. The signs ofcorrectable astigmatism for Vβ>0 and when V 13<0 are opposite to eachother. The absolute value of the amount of correctable astigmatism isincreased with the increase in the absolute value of the voltage Vβ.

Although the above is described with an assumption that Vβ=0 or Vα=0 forsimplicity, however, the voltages Vα and Vβ may be changed within arange in which K times the sum of the absolute values of the voltage Vαand Vβ does not exceed the voltage ΔV. That is, the voltages Vα and Vβare set so that it holds |K·Vα|+|K·Vβ|≦ΔV.

A description will be given of waveforms of the voltages applied to therespective regions of the pattern electrode 18 d of the liquid crystaloptical element 4 for correcting astigmatism in the fourth exemplaryembodiment. Similarly to the waveforms shown in FIGS. 7A to 7E and FIGS.11A to 11E, in-phase rectangular wave voltages having a frequency ofapproximately one kilohertz are applied to the respective regions. Theregion 20 a of the pattern electrode 18 d of the liquid crystal opticalelement 4 is applied with a rectangular wave voltage with an amplitudeof ±V1, and the effective voltage thereof is V1. The regions 20 j and 20n are applied with a rectangular wave voltage with an amplitude of±(V1+Vα), and the effective voltage thereof is (V1+Vα). The regions 20 kand 20 o are applied with a rectangular wave voltage with an amplitudeof ±(V1+Vβ), and the effective voltage thereof is (V1+Vβ). The regions20 l and 20 p are applied with a rectangular wave voltage with anamplitude of ±(V1−Vα), and the effective voltage thereof is (V1−Vα). Theregions 20 m and 20 q are applied with a rectangular wave voltage withan amplitude of ±(V1−Vβ), and the effective voltage thereof is (V1−Vβ).The regions 21 j and 21 n are applied with a rectangular wave voltagewith an amplitude of ±(V1+K·Vα), and the effective voltage thereof is(V1+K·Vα). The regions 21 k and 210 are applied with a rectangular wavevoltage with an amplitude of ±(V1−K·Vβ), and the effective voltagethereof is (V1+K·Vβ). The regions 21 l and 21 p are applied with arectangular wave voltage with an amplitude of ±(V1−K·Vα), and theeffective voltage thereof is (V1−K·Vα). The regions 21 m and 21 q areapplied with a rectangular wave voltage with an amplitude of ±(V1−K·Vβ),and the effective voltage thereof is (V1−K·Vβ). The waveform of thevoltage applied to the entire surface electrode of the liquid crystaloptical element 4 for correcting astigmatism is same as that shown inFIG. 8.

As described above, for the liquid crystal optical element 4 forcorrecting astigmatism, the electrode 24 a in the liquid crystal opticalelement 4 is selected as any of the pattern electrodes 18 a to 18 dhaving a pattern for correcting astigmatism, and the electrode 24 b isformed as an entire surface electrode. Structuring the electrode 24 b inthe liquid crystal optical element 4 as a pattern electrode having apattern for correcting coma aberration or spherical aberration allowsthe liquid crystal optical element 4 to serve as a liquid crystaloptical element for correcting coma aberration or spherical aberrationin addition to astigmatism.

[Fifth Exemplary Embodiment]

Next, a description if given of a fifth exemplary embodiment in whichthe liquid crystal optical element 4 provide correction of comaaberration correction in addition to astigmatism. The liquid crystaloptical element 4 for correcting coma aberration in addition toastigmatism is structured to have a liquid crystal polymer sandwichedbetween two substrates. On the surface of one of the substrates on theliquid crystal polymer side, the pattern electrode 18 a having a patternfor correcting astigmatism is formed, and on the surface of the othersubstrate on the liquid crystal polymer side, a pattern electrode 18 ehaving a pattern for correcting coma aberration is formed.

FIG. 16 is a plan view of the pattern electrode 18 e of the liquidcrystal optical element 4 for correcting coma aberration. In the figure,X and Y axes respectively correspond to the radial and tangentialdirections of the disk 7. The pattern electrode 18 e is divided intofive regions positioned in symmetry with respect to the Y-axis passingthrough the optical axis. Specifically, the pattern electrode 18 e isdivided into: (2) regions 22 b and 22 c spaced apart from each otherwith an islands structure at positions in symmetry with respect to astraight line directed in parallel to the Y axis passing through theoptical axis; a region 22 a provided to surround these regions 22 b and22 c; and (5) regions 22 d and 22 e which are provided outside theregion 22 a in symmetry with respect to the straight line directed inparallel to the Y-axis direction passing through the optical axis, andare respectively positioned on the same side as the regions 22 b and 22c with respect to the straight line directed in parallel to the Y-axisdirection passing through the optical axis. The dotted line in thefigure corresponds to the effective diameter of the objective lens 6.Moreover, sectional views of the liquid crystal optical element 4 aresame as those shown in FIGS. 4A to 4C. It should be noted that theelectrode 24 a is the pattern electrode 18 a having a pattern forcorrecting astigmatism, and that the electrode 24 b is the patternelectrode 18 e having a pattern for correcting coma aberration.

FIGS. 17A to 17D show wave aberration when coma aberration is correctedby the liquid crystal optical element 4. FIGS. 17A to 17D show the waveaberration on the cross section in the X-axis direction passing throughthe optical axis. Solid lines of FIGS. 17A and 17B indicate the waveaberration caused by coma aberration to be corrected. The liquid crystaloptical element drive circuit 15 controls voltages applied to thepattern electrode 18 e of the liquid crystal optical element 4 tothereby generate the wave aberration for correction. Dotted lines ofFIGS. 17A and 17B indicate the wave aberration caused by the liquidcrystal optical element 4 configured to correct the coma aberration.Solid lines of FIGS. 17C and 17D indicate remaining wave aberration whenthe coma aberration is corrected by the liquid crystal optical element4.

When the coefficient of X(X²+Y²), which indicates the wave aberrationcaused by the coma aberration to be corrected, is negative, as shown bya solid line in FIG. 17A, the wave aberration on the cross section inthe X-axis direction passing through the optical axis changes in theform of a cubic function, i.e., to positive values, to negative values,to positive values, and to negative values as it goes from the negativeside to the positive side of the X-axis. When the spherical aberrationis corrected by the liquid crystal optical element 4, as shown by thedotted line in FIG. 17A, the wave aberration caused by the liquidcrystal optical element 4 that corrects the coma aberration on the crosssection in the X-axis direction passing through the optical axis changesin a stair case pattern, i.e., to a negative value, to 0, to a positivevalue, to 0, to the negative value, to 0, and to the positive value asit goes from the negative side to the positive side of the X-axis. Whenthe amount of the coma aberration corrected by the liquid crystaloptical element 4 is optimally determined, the RMS of the remaining waveaberration is minimized after the coma aberration correction. FIG. 17Cshows the remaining wave aberration on the cross section in the X-axisdirection passing through the optical axis in this condition, that is,the superposition of the wave aberration indicated by the solid line andthe wave aberration indicated by the dotted line in FIG. 17A. As isunderstood from FIG. 17C, the absolute value of the remaining waveaberration on the cross section in the X-axis direction passing throughthe optical axis is reduced to around 0.

If the coefficient of X(X²+Y²), which indicates the wave aberrationcaused by the coma aberration to be corrected is positive, as shown by asolid line in FIG. 17B, the wave aberration on the cross section in theX-axis direction passing through the optical axis changes in the form ofa cubic function, i.e., to negative values, to positive values, tonegative values, to positive values as it goes from the negative side tothe positive side of the X-axis. As shown by a dotted line in FIG. 17B,the wave aberration caused by the liquid crystal optical element 4 thatcorrects the coma aberration on the cross section in the X-axisdirection passing through the optical axis changes in a stair casepattern, i.e., to a positive value, to 0, to a negative value, to 0, tothe positive value, to 0, to the negative value, as it goes from thenegative side to the positive side of the X-axis. When the amount of thecoma aberration corrected by the liquid crystal optical element 4 isoptimally determined, the RMS of the remaining wave aberration isminimized after coma aberration correction. FIG. 17D shows the remainingwave aberration on the cross section in the X-axis direction passingthrough the optical path in this condition, that is, the superpositionof the wave aberration indicated by the solid line and the waveaberration indicated by the dotted line in FIG. 17B. As is understoodfrom FIG. 17D, the absolute value of the remaining wave aberration onthe cross section in the X-axis direction passing through the opticalaxis is reduced to around 0.

The relationship between the voltages applied to the electrode of theliquid crystal optical element 4 and the phase of light transmittedthrough the liquid crystal optical element 4 is same as shown in FIG. 5.Here, for simplicity, it is assumed that all the regions of the patternelectrode 18 a of the liquid crystal optical element 4 are applied withthe voltage V1. Moreover, out of the regions of the pattern electrode 18e of the liquid crystal optical element 4, at least one region, forexample, the region 22 a is defined as a reference region, and thevoltage applied to the reference region is defined as V2. It should benoted that V1+V2=V0. That is, the voltage between the pattern electrode18 a and the reference region of the pattern electrode 18 e is V0, andthe phase of light transmitted through the liquid crystal opticalelement 4 in the reference region is φ0. If the voltage applied to aregion other than the reference region out of the respective regions ofthe pattern electrode 18 e of the liquid crystal optical element 4 islower (the absolute value is larger) than V2, the voltage between thepattern electrode 18 a and the pattern electrode 18 e is increased aboveV0, and the phase of light transmitted through the liquid crystaloptical element 4 in this region is advanced with respect to φ0. Thatis, the light transmitted through this region of the liquid crystaloptical element 4 experiences positive wave aberration with respect tothe light transmitted through the reference region of the liquid crystaloptical element 4. On the other hand, if the voltage applied to a regionother than the reference region out of the respective regions of thepattern electrode 18 e of the liquid crystal optical element 4 is higher(the absolute value thereof is smaller) than V2, the voltage between thepattern electrode 18 a and the pattern electrode 18 e is reduced belowV0, and the phase of the light transmitted through this region of theliquid crystal optical element 4 is delayed with respect to φ0. That is,the light transmitted through this region of the liquid crystal opticalelement 4 experiences negative wave aberration with respect to the lighttransmitted through the reference region of the liquid crystal opticalelement 4.

FIG. 18 shows the relationship between the regions of the patternelectrode 18 e of the liquid crystal optical element 4 for correctingcoma aberration and the voltages applied to the respective regions. Forsimplicity, it is assumed that the voltages applied to the respectiveregions of the pattern electrode 18 a of the liquid crystal opticalelement 4 are all set to V1. That is, it is assumed that Vα=0 and Vβ=0.Moreover, out of the regions of the pattern electrode 18 e of the liquidcrystal optical element 4, the region 22 a is defined as a referenceregion, and the voltage applied to the reference region 22 a is definedas V2. It should be noted that V1+V2=V0. The voltage applied to theregions 22 b and 22 e is V2−VΥ, and the voltage applied to the regions22 c and 22 d is V2+VΥ.

For VΥ>0, light transmitted through the regions 22 b and 22 e of theliquid crystal optical element 4 is delayed in phase with respect tolight transmitted through the reference region 22 a of the liquidcrystal optical element 4, and light transmitted through the regions 22c and 22 d of the liquid crystal optical element 4 is advanced in phase.For VΥ<0, on the other hand, light transmitted through the regions 22 band 22 e of the liquid crystal optical element 4 is advanced in phasewith respect to the light transmitted through the reference region 22 aof the liquid crystal optical element 4, and the light transmittedthrough the regions 22 c and 22 d of the liquid crystal optical element4 is delayed in phase. As thus described, a change in the voltage VΥcauses a change in the correction amount of coma aberration. The signsof correctable coma aberration for VΥ>0 and VΥ<0 are opposite to eachother. The absolute value of the amount of correctable coma aberrationis increased with the increase in the absolute value of the voltage VΥ.

Although the above is described with an assumption that Vα=0 or Vβ=0,for simplicity, the voltages Vα, Vβ, and VΥ may be actually changedwithin a range in which the sum of the respective absolute values ofthese voltages does not exceed ΔV. That is, the voltages Vα, Vβ, and VΥare set so that it holds |Vα|+|Vβ|+|VΥ|≦ΔV.

Although a description is given of a case in which the pattern electrode18 e of the liquid crystal optical element 4 has a pattern forcorrecting coma aberration in the X-axis direction with reference toFIGS. 16 to 18, the same applies to a case in which the patternelectrode 18 e of the liquid crystal optical element 4 has a pattern forcorrecting coma aberration in the Y-axis direction.

In the fifth exemplary embodiment, the electrode 24 a of the liquidcrystal optical element 4 is the pattern electrode 18 a having a patternfor correcting astigmatism, and the electrode 24 b is the patternelectrode 18 e having a pattern for correcting coma aberration.Contrarily, in a possible exemplary embodiment, the electrode 24 a ofthe liquid crystal optical element 4 may be any of the patternelectrodes 18 b to 18 d having a pattern for correcting astigmatism andthe electrode 24 b may be the pattern electrode 18 e having a patternfor correcting coma aberration.

[Sixth Exemplary Embodiment]

Next, a description will be given of a sixth exemplary embodiment inwhich the liquid crystal optical element 4 corrects spherical aberrationin addition to astigmatism. The liquid crystal optical element 4 forcorrecting spherical aberration in addition to astigmatism is structuredto have a liquid crystal polymer sandwiched between two substrates. Onthe surface of one of the substrates on the liquid crystal polymer side,the pattern electrode 18 a having a pattern for correcting astigmatismis formed, and on the surface of the other substrate on the liquidcrystal polymer side, a pattern electrode 18 f having a pattern forcorrecting spherical aberration is formed.

FIG. 19 is a plan view of the pattern electrode 18 f of the liquidcrystal optical element 4 for correcting spherical aberration. In thefigure, X and Y axes respectively correspond to the radial andtangential directions of the disk 7. The pattern electrode 18 f isdivided into five regions by four circles with the optical axis servingas center. Specifically, the pattern electrode 18 f is divided intoregions 22 f to 22 j with the optical axis as center. The dotted line inthe figure corresponds to the effective diameter of the objective lens6. Moreover, sectional views of the liquid crystal optical element 4 aresame as shown in FIGS. 4A to 4C. It should be noted that the electrode24 a is the pattern electrode 18 a having a pattern for correctingastigmatism, and that the electrode 24 b is the pattern electrode 18 fhaving a pattern for correcting spherical aberration.

FIGS. 20A to 20D show wave aberration when spherical aberration iscorrected by the liquid crystal optical element 4. FIGS. 20A to 20D showthe wave aberration on the cross section in the X-axis direction passingthrough the optical axis. It should be noted that wave aberration on thecross section in an arbitrary direction passing through the optical axisis equal to the wave aberration on the cross section in the X-axisdirection passing through the optical axis. Solid lines of FIGS. 20A and20B show the wave aberration caused by spherical aberration to becorrected. The liquid crystal optical element drive circuit 15 controlsvoltages applied to the pattern electrode 18 f of the liquid crystaloptical element 4 to thereby generate the wave aberration forcorrection. Dotted lines of FIGS. 20A and 20B show the wave aberrationcaused by the liquid crystal optical element 4 that corrects thisspherical aberration. Solid lines of FIGS. 20C and 20D show remainingwave aberration when the spherical aberration is corrected by the liquidcrystal optical element 4.

If the coefficient of (X²+Y²)², indicating the wave aberration caused bythe spherical aberration to be corrected, is negative, as shown by thesolid line in FIG. 20A, the wave aberration on the cross section in theX-axis direction passing through the optical axis changes in the form ofa quartic function, i.e., to negative values, to positive values, tonegative values, to positive values, to negative values as it goes fromthe negative side to the positive side of the X-axis. When thisspherical aberration is corrected by the liquid crystal optical element4, as shown by the dotted line in FIG. 20A, the wave aberration causedby the liquid crystal optical element 4 on the cross section in theX-axis direction passing through the optical axis changes in a staircase pattern, i.e., to a positive value, to 0, to a negative value, to0, to the positive value, to 0, to the negative value, to 0, to thepositive as it goes from the negative side to the positive side of theX-axis. When the amount of the spherical aberration corrected by theliquid crystal optical element 4 is optimally determined, the RMS of theremaining wave aberration is minimized after the spherical aberrationcorrection. FIG. 20C shows the remaining wave aberration on the crosssection in the X-axis direction passing through the optical axis in thiscase, that is, the sum of the wave aberration indicated by the solidline and the wave aberration indicated by the dotted line in FIG. 20A.As is understood from FIG. 20C, the absolute value of the remaining waveaberration on the cross section in the X-axis direction passing throughthe optical axis is reduced to around 0.

When the coefficient of (X²+Y²)², which indicates the wave aberrationcaused by the spherical aberration to be corrected, is positive, asshown by the solid line in FIG. 20B, the wave aberration on the crosssection in the X-axis direction passing through the optical axis changesin the form of a quartic function, i.e., to positive values, negativevalues, to positive values, to negative values, and to positive valuesas it goes from the negative side to the positive side of the X-axis. Asshown by the dotted line in FIG. 20B, the wave aberration caused by theliquid crystal optical element 4 that corrects the spherical aberrationon the cross section in the X-axis direction passing through the opticalaxis changes in a stair case pattern, i.e., to a negative value, to 0,to a positive value, to 0, to the negative value, to 0, to the positivevalue, to 0, to the negative value as it goes from the negative side tothe positive side of the X-axis. When the amount of the sphericalaberration corrected by the liquid crystal optical element 4 isoptimally determined, the RMS of the remaining wave aberration isminimized after the spherical aberration correction. FIG. 20D shows theremaining wave aberration on the cross section in the X-axis directionpassing through the optical axis in this condition, that is, the superposition of the wave aberration indicated by the solid line and the waveaberration indicated by the dotted line in FIG. 20B. As is understoodfrom FIG. 20D, the absolute value of the remaining wave aberration onthe cross section in the X-axis direction passing through the opticalaxis is reduced to around 0.

The relationship between the voltages applied to the electrode of theliquid crystal optical element 4 and the phase of light transmittedthrough the liquid crystal optical element 4 is same as that shown inFIG. 5. Here, it is assumed for simplicity that all the regions of thepattern electrode 18 a of the liquid crystal optical element 4 areapplied with the voltage V1. Moreover, out of the regions of the patternelectrode 18 f of the liquid crystal optical element 4, at least oneregion, for example, the region 22 i is defined as a reference region,and the voltage applied to the reference region is defined as V3. Notethat V1+V3=V0. That is, the voltage between the pattern electrode 18 aand the reference region of the pattern electrode 18 f is V0, and thephase of light transmitted through the reference region of the liquidcrystal optical element 4 is φ0. If the voltage applied to a regionother than the reference region out of the respective regions of thepattern electrode 18 f of the liquid crystal optical element 4 is lower(its absolute value is larger) than V3, the voltage between the patternelectrode 18 a and the pattern electrode 18 f is increased above thanV0, and the phase of light transmitted through this region of the liquidcrystal optical element 4 is advanced with respect to φ0. That is, thelight transmitted through this region of the liquid crystal opticalelement 4 experiences positive wave aberration with respect to the lighttransmitted through the liquid crystal optical element 4. On the otherhand, when the voltage applied to a region other than the referenceregion out of the respective regions of the pattern electrode 18 f ofthe liquid crystal optical element 4 is higher (its absolute value issmaller) than V3, the voltage between the pattern electrode 18 a and thepattern electrode 18 f is increased above V0, and the phase of the lighttransmitted through this region of the liquid crystal optical element 4is delayed with respect to φ0. That is, the light transmitted throughthis region of the liquid crystal optical element 4 experiences negativewave aberration with respect to the light transmitted through thereference region of the liquid crystal optical element 4.

FIG. 21 shows the relationship between the regions of the patternelectrode 18 f of the liquid crystal optical element 4 for correctingspherical aberration and the voltages applied to the respective regions.It is assumed for simplicity that the voltages applied to the respectiveregions of the pattern electrode 18 a of the liquid crystal opticalelement 4 are all set to V1. That is, it is assumed that Vα=0 and Vβ=0.Moreover, out of the regions of the pattern electrode 18 f of the liquidcrystal optical element 4, the region 22 g and the region 22 i aredefined as reference regions, and the voltage applied to the referenceregions is defined as V3. Note that V1+V3=V0. The voltage applied to theregion 22 f and the region 22 j is V3+Vδ, and the voltage applied to theregion 22 h is V3−Vδ.

For Vδ>0, light transmitted through the regions 22 f and 22 j of theliquid crystal optical element 4 is advanced in phase with respect tolight transmitted through the reference regions 22 g and 22 i of theliquid crystal optical element 4, and light transmitted through theregion 22 h of the liquid crystal optical element 4 is delayed in phase.Moreover, for Vδ<0, light transmitted through the regions 22 f and 22 jof the liquid crystal optical element 4 is delayed in phase with respectto the light transmitted through the reference regions 22 g and 22 i ofthe liquid crystal optical element 4, and light transmitted through theregion 22 h of the liquid crystal optical element 4 is advanced inphase. As thus described, a change in the voltage Vδ causes a change inthe correction amount of spherical aberration. The signs of correctablespherical aberration when Vδ>0 and when Vδ<0 are opposite to each other.The absolute value of the amount of correctable spherical aberration isincreased with the increase in the absolute value of the voltage Vδ.

Although the above is described with an assumption that Vα=0 or Vβ=0 forsimplicity, the voltages Vα, Vβ, and Vδ may be actually changed within arange such that the sum of the respective absolute values of thesevoltages does not exceed ΔV. That is, the voltages Vα, Vβ, and Vδ areset so that it holds |Vα|+|Vβ|+|Vδ|≦ΔV.

In the sixth exemplary embodiment, the electrode 24 a of the liquidcrystal optical element 4 is the pattern electrode 18 a having a patternfor correcting astigmatism, and the electrode 24 b is the patternelectrode 18 f having a pattern for correcting spherical aberration.Contrarily, in possible exemplary embodiments, the electrode 24 a of theliquid crystal optical element 4 may be any of the pattern electrodes 18b to 18 d having a pattern for correcting astigmatism, and the electrode24 b may be the pattern electrode 18 f having a pattern for correctingspherical aberration.

FIGS. 22A to 22C show waveforms of the voltages applied to therespective regions of the pattern electrodes 18 e or 18 f of the liquidcrystal optical element 4 for correcting coma aberration or sphericalaberration. In FIGS. 22A to 22C, the horizontal axis denotes time, andthe vertical axis denotes the voltage. Long-term application of a dcvoltage to the electrode of the liquid crystal optical element causesdestruction of the liquid crystal polymer. Thus, an ac voltage isactually applied. As shown in FIGS. 22A to 22C, the respective regionsof the pattern electrodes 18 e and 18 f are applied with in-phaserectangular wave voltages having a frequency of approximately onekilohertz. These rectangular voltages are opposite in phase to therectangular voltages applied to the regions 19 a to 19 i of the patternelectrode 18 a. As shown in FIG. 22A, the region 22 a of the patternelectrode 18 e of the liquid crystal optical element 4 is applied with arectangular wave voltage with an amplitude of ±V2, and the effectivevoltage thereof is V2. As shown in FIG. 22B, the regions 22 b and 22 eare applied with a rectangular wave voltage with an amplitude of±(V2−VΥ), and the effective voltage thereof is (V2−VΥ). As shown in FIG.22C, the regions 22 c and 22 d are applied with a rectangular wavevoltage with an amplitude of ±(V2+VΥ), and the effective voltage thereofis (V2+VΥ).

Moreover, as shown in FIG. 22A, the regions 22 g and 22 i of the patternelectrode 18 f of the liquid crystal optical element 4 are applied witha rectangular wave voltage with an amplitude of ±V3, and the effectivevoltage thereof is V3. As shown in FIG. 22B, the region 22 h is appliedwith a rectangular wave voltage with an amplitude of ±(V3−Vδ), and theeffective voltage thereof is (V3−Vδ). As shown in FIG. 22C, the regions22 f and 22 j are applied with a rectangular wave voltage with anamplitude of ±(V3+Vδ), and the effective voltage thereof is (V3+Vδ). Thewaveforms of the voltages applied to the regions 22 f to 22 j are equalin phase, but the phase thereof is opposite to the waveforms of thevoltages applied to the regions 19 a to 19 i of the pattern electrode 18a of the liquid crystal optical element 4.

Next, a description is given of an optical informationrecording/reproducing method according to an exemplary embodiment of thepresent invention. In correcting astigmatism by the liquid crystaloptical element 4, the optimum value of the voltage Vα is determinedwith the voltage Vβ fixed at a predetermined value, so that the qualityevaluation index of the reproduced signal from the optical recordingmedium is best improved. In addition, the optimum value of the voltageVβ is determined with the voltage Vα fixed at a predetermined value, sothat the quality evaluation index of the reproduced signal from theoptical recording medium is best improved. It should be noted thatwhichever of the step for determining the optimum value of the voltageVα with the voltage Vβ fixed at the predetermined value and the step fordetermining the optimum value of the voltage Vβ with the voltage Vαfixed at the predetermined value may be carried out first.

Determining the optimum value of the voltage Vα with the voltage Vβfixed at the predetermined value so that the quality evaluation index ofthe reproduced signal from the optical recording medium is best improvedallows obtaining the optimum correction amount for the astigmatismbetween the 0° and 90° directions.

Moreover, determining the optimum value of the voltage Vβ with thevoltage Vα fixed at a predetermined value so that the quality evaluationindex of the reproduced signal from the optical recording medium is bestimproved allows obtaining the optimum correction amount for theastigmatism between the 45° and 135° directions. After the combinationof the optimum value of the voltage Vα and the optimum value of thevoltage Vβ is determined, the rectangular voltages are applied to therespective regions of the pattern electrode 18 of the liquid crystaloptical element 4 on the basis of this combination. Specifically, therespective regions of the pattern electrode 18 a of the liquid crystaloptical element 4 are applied with rectangular voltages having theeffective voltages shown in FIG. 6. Alternatively, the respectiveregions of the pattern electrode 18 b of the liquid crystal opticalelement 4 are applied with rectangular voltages having the effectivevoltages shown in FIG. 10. Alternatively, the respective regions of thepattern electrode 18 c of the liquid crystal optical element 4 areapplied with rectangular voltages having the effective voltages shown inFIG. 13. Alternatively, the respective regions of the pattern electrode18 d of the liquid crystal optical element 4 are applied withrectangular voltages having the effective voltages shown in FIG. 15 areapplied. This allows correcting the astigmatism between the 0° and 90°directions and the astigmatism between the 45° and 135° directions,simultaneously. That is, astigmatism of an arbitrary direction andamount can be corrected. Under this condition, the quality of thereproduced signal from the optical recording medium is best improved.

When coma aberration or spherical aberration is corrected in addition toastigmatism, the liquid crystal optical element 4 is configured toinclude the pattern electrodes 18 a and 18 e or the pattern electrodes18 a and 18 f as the electrodes 24 a and 24 b. First, the optimum valueof the voltage Vα is determined with the voltage Vβ and the voltage VΥor the voltage Vδ fixed at predetermined values so that the qualityevaluation index of the reproduced signal from the optical recordingmedium is best improved. With the voltage Vα and the voltage VΥ or thevoltage Vδ fixed at predetermined values, the optimum value of thevoltage Vβ is determined so that the best quality evaluation index of areproduced signal from the optical recording medium is provided. Next,the optimum value of the voltage VΥ or the voltage Vδ is determined withthe voltages Vα and Vβ fixed at predetermined values, so that thequality evaluation index of a reproduced signal from the opticalrecording medium is best improved. Here, any of the step for determiningthe optimum value of the voltage Vα with the voltage Vβ and the voltageVΥ or the voltage Vδ fixed at the predetermined values, the step fordetermining the optimum value of the voltage Vβ with the voltage Vα andthe voltage VΥ or the voltage Vδ fixed at the predetermined values, andthe step for determining the optimum value of the voltage VΥ or thevoltage Vδ with the voltages Vα and Vβ fixed at the predetermined valuesmay be carried out first.

Determining the optimum value of the voltage Vα with the voltage Vβ andthe voltage VΥ or the voltage Vδ fixed at the predetermined values sothat the quality evaluation index of the reproduced signal from theoptical recording medium is best improved, allows obtaining the optimumcorrection amount for the astigmatism between 0° and 90° directions. Inaddition, determining the optimum value of the voltage Vβ with thevoltage Vα and the voltage VΥ or the voltage Vδ fixed at thepredetermined values so that the quality evaluation index of thereproduced signal from the optical recording medium is best improvedallows obtaining the optimum correction amount for the astigmatismbetween the 45° and 135° directions. Further, determining the optimumvalue of the voltage VΥ or the voltage Vδ with the voltages Vα and Vβfixed at the predetermined values so that the quality evaluation indexof the reproduced signal from the optical recording medium is bestimproved allows obtaining the optimum correction amount for comaaberration or spherical aberration.

After the combination of the optimum value of the voltages Vα, Vβ, andthe optimum value of the voltage VΥ or Vδ is determined, the rectangularvoltages are applied to the respective regions of the pattern electrode18 of the liquid crystal optical element 4 on the basis of thedetermined combination. Specifically, the respective regions of thepattern electrode 18 a of the liquid crystal optical element 4 areapplied with rectangular voltages having the effective voltages shown inFIG. 6. Alternatively, the respective regions of the pattern electrode18 b of the liquid crystal optical element 4 are applied withrectangular voltages having the effective voltages shown in FIG. 10.Alternatively, the respective regions of the pattern electrode 18 c ofthe liquid crystal optical element 4 are applied with rectangularvoltages having the effective voltages shown in FIG. 13. Alternatively,the respective regions of the pattern electrode 18 d of the liquidcrystal optical element 4 are applied with rectangular voltages havingthe effective voltages shown in FIG. 15. In addition, the respectiveregions of the pattern electrode 18 e of the liquid crystal opticalelement 4 are applied with rectangular voltages having the effectivevoltages shown in FIG. 18, or the respective regions of the patternelectrode 18 f of the liquid crystal optical element 4 are applied withrectangular voltages having the effective voltages shown in FIG. 21. Asa result, the astigmatism between the 0° and 90° directions, theastigmatism between the 45° and 135° directions, and coma aberration orspherical aberration are corrected simultaneously. That is, astigmatismand coma aberration or spherical aberration of an arbitrary directionand amount can be corrected. Under this condition, the quality of thereproduced signal from the optical recording medium is best improved.

As the quality evaluation index of the reproduced signal, for example,any of an amplitude of a reproduced signal, jitter, PRSNR (PartialResponse Signal to Noise Ratio), an error rate and so on may be used.

FIGS. 24A to 27 show measurement examples of the quality evaluationindex of the reproduced signal by the optical informationrecording/reproducing method of the present invention.

FIGS. 24A and 24B show the measurement examples of in connection withthe correction of astigmatism in the first exemplary embodiment. Theliquid crystal optical element 4 for correcting astigmatism is used, theoptical recording medium is an HD DVD-ROM, and the quality evaluationindex of the reproduced signal is the PRSNR. In FIG. 24A, the voltage Vαis varied with the voltage the Vβ fixed at 0 volts to measure therelationship between the voltage Vα and PRSNR. As is understood FromFIG. 24A, the voltage Vα to minimize the PRSNR is 0.04 volts, and thevalue of the PRSNR in this condition is approximately 15. In FIG. 24B,the voltage Vβ is varied with the voltage Vα fixed at 0.04 volts asobtained in FIG. 24A to measure the relationship between the voltage Vβand the PRSNR. As is understood from FIG. 24B, the voltage Vβ tominimize the PRSNR is 0 volts, and the value of the PRSNR in thiscondition is approximately 15. In this case, the combination of theoptimum values of the voltages Vα and Vβ to minimize the PRSN is 0.04volts and 0 volts.

FIGS. 25A and 25B show the examples of measurement in connection withthe correction of astigmatism in the first exemplary embodiment when thejitter is used as the quality evaluation index of the reproduced signal.The liquid crystal optical element 4 for correcting astigmatism is used,and the optical recording medium is DVD-ROM. In FIG. 25A the voltage Vαis varied with the voltage the Vβ fixed at 0 volts to measure therelationship between the voltage Vα and the jitter. As is understoodfrom FIG. 25A, the voltage Vα to minimize the jitter is 0.07 volts, andthe value of the jitter in this condition is approximately 6.5%. In FIG.25B, the voltage Vβ is varied with the voltage Vα fixed at 0.07 volts asobtained in FIG. 25A, to measure the relationship between the voltage Vβand the jitter. As is understood from FIG. 25B, the voltage Vβ tominimize the jitter is 0.02 volts, and the jitter in this condition isapproximately 6.5%. In this case, the combination of the optimum valuesof the voltages Vα and Vβ to minimize the jitter is 0.07 volts and 0.02volts.

FIG. 26 shows a measurement example in connection with the correction ofcoma aberration in the fifth exemplary embodiment. The liquid crystaloptical element for correcting coma aberration in addition toastigmatism is used, the optical recording medium is an HD DVD-ROM, andthe quality evaluation index of the reproduced signal is the PRSNR. InFIG. 26, the voltage VΥ is varied to measure the relationship betweenthe voltage VΥ and the PRSNR with the voltage Vα fixed at 0.04 volts asobtained in FIG. 24A and with the voltage Vβ fixed at 0 volts asobtained in FIG. 24B. As is understood from FIG. 26, the voltage VΥ tominimize the PRSNR is 0.04 volts, and the value of the PRSNR in thiscondition is approximately 16. In this case, the combination of theoptimum values of the voltages Vα, Vβ, and VΥ to minimize the PRSNR is0.04 volts, 0 volts, and 0.04 volts.

FIG. 27 shows a measurement example in connection with the correction ofcoma aberration in the fifth exemplary embodiment when the jitter isused as the quality evaluation index of the reproduced signal. Theliquid crystal optical element 4 for correcting coma aberration inaddition to astigmatism is used, and the optical recording medium is aDVD-ROM. In FIG. 27, the voltage VΥ is varied to measure relationshipbetween the voltage VΥ and the jitter with the voltage Vα fixed at 0.07volts as obtained in FIG. 25A and with the voltage the Vβ fixed at 0.02volts as obtained in FIG. 25B. As is understood from FIG. 27, thevoltage VΥ to minimize the jitter is −0.04 volts, and the value of thejitter in this condition is approximately 6.5%. In this case, thecombination of the optimum values of the voltages Vα, Vβ, and VΥ tominimize the jitter is 0.07 volts, 0.02 volts, and −0.04 volts. Thecontrol of the voltages applied to the respective regions of the patternelectrodes of the liquid crystal optical element as thus describedallows determining in short time the applied voltages such that thequality of the reproduced signal is best improved.

1. An optical information recording/reproducing apparatus comprising: anoptical head including: a light source; an objective lens focusing anemitted light emitted from said light source on an optical recordingmedium; a photo-detector receiving a reflected light generated by saidemitted light being reflected by said optical recording medium; a lightsplitter splitting forward light directed from said light source to saidobjective lens and backward light directed from said objective lens tosaid photo-detector; and a liquid crystal optical element which isprovided in a light path of said forward light and includes a liquidcrystal polymer layer extending perpendicularly to an optical axis; anda liquid crystal optical element drive unit driving said liquid crystaloptical element, wherein said liquid crystal optical element includes afirst pattern electrode provided on one side of said liquid crystalpolymer layer in said optical axis direction and divided into aplurality of regions, wherein said first pattern electrode includes: afirst region provided to surround said optical axis, a set of eightpartition regions provided outside said first region and defined so asto divide a circumference into eight segments with respect to saidoptical axis, said set of eight partition regions including second toninth regions sequentially arranged, wherein said liquid crystal opticalelement drive unit is configured: to apply a first effective voltage tosaid first region; to apply a second effective voltage to said secondand sixth regions; to apply a third effective voltage to said third andseventh regions; to apply a fourth effective voltage to said fourth andeighth regions; to apply a fifth effective voltage to said fifth andninth regions; and wherein an average of said second and fourtheffective voltages and an average of said third and fifth effectivevoltages are equal to said first effective voltage; wherein said firstpattern electrode further includes: a second set of eight partitionregions which are provided outside said second to ninth regions anddefined so as to divide a circumference into eight segments with respectto said optical axis, said second set of eight partition regionsincluding tenth to seventeenth regions sequentially arranged, whereinsaid liquid crystal optical element drive unit is further configured: toapply a sixth effective voltage to said tenth and fourteenth regions; toapply a seventh effective voltage to said eleventh and fifteenthregions; to apply an eighth effective voltage to said twelfth andsixteenth regions; to apply a ninth effective voltage to said thirteenthand seventeenth regions; and wherein an average of said sixth and eightheffective voltages and an average of said seventh and ninth voltages areequal to said first effective voltage.
 2. The optical informationrecording/reproducing apparatus according to claim 1, wherein adifference between said sixth and first effective voltages is K times adifference between said second and first effective voltages, where K isa constant larger than one, wherein a difference between said seventhand first effective voltages is K times a difference between said thirdand first effective voltages, wherein a difference between said eighthand first effective voltages is K times a difference between said fourthand first effective voltages, and wherein a difference between saidninth and first effective voltages is K times a difference between saidfifth and first effective voltages.
 3. The optical informationrecording/reproducing apparatus according to claim 1, wherein said firsteffective voltage is V1, said second effective voltage is V1+Vα+Vβ, saidthird effective voltage is V1−Vα+Vβ, said fourth effective voltage isV1−Vα−Vβ, said fifth effective voltage is V1+Vα−Vβ, said sixth effectivevoltage is V1+K·Vα+K·Vβ, said seventh effective voltage is V1−K·Vα+K·Vβ,said eighth effective voltage is V1−K·Vα−K·Vβ, and said ninth effectivevoltage is V1+K·Vα−K·Vβ, where V1 is a first reference voltage value, Vαis a first voltage value, and Vβ is a second voltage value.
 4. Theoptical information recording/reproducing apparatus according to claim1, wherein said first effective voltage is V1, said second effectivevoltage is V1+Vα, said third effective voltage is v1+Vβ, said fourtheffective voltage is V1−Vα, said fifth effective voltage is V1−Vβ, saidsixth effective voltage is V1+K·Vα, said seventh effective voltage isV1+K·Vβ, said eighth effective voltage is V1−K·Vα, and said nintheffective voltage is V1−K·Vβ, where V1 is a first reference voltagevalue, Vα is a first voltage value, and Vβ is a second voltage value. 5.The optical information recording/reproducing apparatus according toclaim 1, wherein said liquid crystal optical element further includes asecond pattern electrode provided on another side of said liquid crystalpolymer layer in said optical axis direction, positioned opposed to saidfirst pattern electrode, and divided into a plurality of regions,wherein said second pattern electrode includes: eighteenth andnineteenth regions provided apart from each other in an island structureat positions approximately in symmetry with respect to a straight linepassing through said optical axis and directed in a predetermineddirection, a twentieth region provided outside said eighteenth andnineteenth regions to surround said eighteenth and nineteenth regions,and twenty-first and twenty-second regions provided outside saidtwentieth region approximately in symmetry with said straight line on asame side of said eighteenth and nineteenth regions with respect to saidstraight line, wherein said liquid crystal optical element drive unit isfurther configured: to apply a tenth effective voltage to said twentiethregion; to apply an eleventh effective voltage to said eighteenth andtwenty-second regions; and to apply a twelfth effective voltage to saidnineteenth and twenty-first regions; and wherein an average of saideleventh and twelfth effective voltages is equal to said tenth effectivevoltage.
 6. An optical information recording/reproducing methodcomprising: driving a liquid crystal optical element disposed in anoptical path of forward light within an optical head, provided with aliquid crystal polymer layer extending perpendicularly to an opticalaxis and including a first pattern electrode positioned on one side ofsaid liquid crystal polymer layer in said optical axis direction;generating a reproduced signal based on backward light reflected by anoptical recording medium; and controlling drive of said liquid crystaloptical element in said driving so that a quality evaluation index ofsaid reproduced signal is best improved, wherein said first patternelectrode includes: a first region provided to surround said opticalaxis, a set of eight partition regions provided outside said firstregion and defined so as to divide a circumference into eight segmentswith respect to said optical axis, said set of eight partition regionsincluding second to ninth regions sequentially arranged, wherein saiddriving includes: applying an effective voltage of V1 to said firstregion, applying an effective voltage of V1+Vα+Vβ to said second andsixth regions; applying an effective voltage of V1−Vα+Vβ to said thirdand seventh regions; applying an effective voltage of V1−Vα−Vβ to saidfourth and eighth regions; applying an effective voltage of V1+Vα−Vβ tosaid fifth and ninth regions, where V1 is a first reference voltagevalue, Vα is a first voltage value, and Vβ is a second voltage value;wherein said first pattern electrode further includes: a second set ofeight partition regions which are provided outside said second to ninthregions and defined so as to divide a circumference into eight segmentswith respect to said optical axis, said second set of eight partitionregions including tenth to seventeenth regions sequentially arranged,wherein said driving further includes: applying an effective voltage ofV1+K·Vα+K·Vβ to said tenth and fourteenth regions; applying an effectivevoltage of V1−K·Vα+K·Vβ to said eleventh and fifteenth regions; applyingan effective voltage of V1−K·Vβ−K·Vβ to said twelfth and sixteenthregions; and applying an effective voltage of V1+K·Vα−K·Vβ to saidthirteenth and seventeenth regions, where K is a constant larger thanone.
 7. An optical information recording/reproducing method comprising:driving a liquid crystal optical element disposed in an optical path offorward light within an optical head, provided with a liquid crystalpolymer layer extending perpendicularly to an optical axis and includinga first pattern electrode positioned on one side of said liquid crystalpolymer layer in said optical axis direction; generating a reproducedsignal based on backward light reflected by an optical recording medium;and controlling drive of said liquid crystal optical element in saiddriving so that a quality evaluation index of said reproduced signal isbest improved, wherein said first pattern electrode includes: a firstregion provided to surround said optical axis, a set of eight partitionregions provided outside said first region and defined so as to divide acircumference into eight segments with respect to said optical axis,said set of eight partition regions including second to ninth regionssequentially arranged, wherein said driving includes: applying aneffective voltage of V1 to said first region; applying an effectivevoltage of V1+Vα to said second and sixth regions; applying an effectivevoltage of V1+Vβ to said third and seventh regions; applying aneffective voltage of V1−Vα to said fourth and eighth regions; applyingan effective voltage of V1−Vβ to said fifth and ninth regions, where V1is a first reference voltage value, Vα is a first voltage value, and Vβis a second voltage value; wherein said first pattern electrode furtherincludes: a second set of eight partition regions which are providedoutside said second to ninth regions and defined so as to divide acircumference into eight segments with respect to said optical axis,said second set of eight partition regions including tenth toseventeenth regions sequentially arranged, wherein said driving furtherincludes: applying an effective voltage of V1+K·Vα to said tenth andfourteenth regions; applying an effective voltage of V1+K·Vβ to saideleventh and fifteenth regions, applying an effective voltage of V1−K·Vαto said twelfth and sixteenth regions; and applying an effective voltageof V1−K·Vβ to said thirteenth and seventeenth regions.
 8. The opticalinformation recording/reproducing method according to claim 6, whereinsaid control step includes: a step of determining an optimum value ofsaid voltage Vα with said voltage Vβ fixed at a predetermined value sothat said quality evaluation index is best improved; and a step ofdetermining an optimum value of said voltage Vβ with said voltage Vαfixed at a predetermined value so that said quality evaluation index isbest improved.
 9. The optical information recording/reproducing methodaccording to claim 6, wherein said liquid crystal optical elementfurther includes a second pattern electrode provided on another side ofsaid liquid crystal polymer layer in said optical axis direction,positioned opposed to said first pattern electrode, and divided into aplurality of regions, wherein said second pattern electrode includes:eighteenth and nineteenth regions provided apart from each other in anisland structure at positions approximately in symmetry with respect toa straight line passing through said optical axis and directed in apredetermined direction; a twentieth region provided outside saideighteenth and nineteenth regions to surround said eighteenth andnineteenth regions; and twenty-first and twenty-second regions providedoutside said twentieth region approximately in symmetry with saidstraight line on a same side of said eighteenth and nineteenth regionswith respect to said straight line, wherein said driving furtherincludes: applying an effective voltage of V2 to said twentieth region;applying an effective voltage of V2−VΥ to said eighteenth andtwenty-second regions, and applying an effective voltage of V2+VΥ tosaid nineteenth and twenty-first regions, where V2 is a second referencevoltage value different from said first reference voltage value, and VΥis a third voltage value.
 10. The optical informationrecording/reproducing method according to claim 9, wherein saidcontrolling further includes determining an optimum value of saidvoltage VΥ with said voltages Vα and Vβ fixed at predetermined values sothat said quality evaluation index is best improved.
 11. The opticalinformation recording/reproducing method according to claim 6, whereinsaid liquid crystal optical element further includes a third patternelectrode provided on another side of said liquid crystal polymer layerin said optical axis direction, positioned opposed to said first patternelectrode, and divided into eighteenth and twenty-second regionsarranged in order from inside to outside to surround said optical axis,wherein said driving further includes: applying an effective voltage ofV3 to said nineteenth and twenty-first regions; applying an effectivevoltage of V3−Vδ to said twentieth region, and applying an effectivevoltage of V3+Vδ to said eighteenth and twenty-second regions, where V3is a third reference voltage value different from said first referencevoltage value, and Vδ is a fourth voltage value.
 12. The opticalinformation recording/reproducing method according to claim 11, whereinsaid further includes a step of determining an optimum value of saidvoltage Vδ with said voltages Vα and Vβ fixed at predetermined values sothat said quality evaluation index is best improved.
 13. An opticalinformation recording/reproducing apparatus comprising: an optical headincluding: a light source; an objective lens focusing an emitted lightemitted from said light source on an optical recording medium; aphoto-detector receiving a reflected light generated by said emittedlight being reflected by said optical recording medium; a light splittersplitting forward light directed from said light source to saidobjective lens and backward light directed from said objective lens tosaid photo-detector; and a liquid crystal optical element which isprovided in a light path of said forward light and includes a liquidcrystal polymer layer extending perpendicularly to an optical axis; anda liquid crystal optical element drive unit driving said liquid crystaloptical element, wherein said liquid crystal optical element includes afirst pattern electrode provided on one side of said liquid crystalpolymer layer in said optical axis direction and divided into aplurality of regions, wherein said first pattern electrode includes: afirst region provided to surround said optical axis, a set of eightpartition regions provided outside said first region and defined so asto divide a circumference into eight segments with respect to saidoptical axis, said set of eight partition regions including second toninth regions sequentially arranged, wherein said liquid crystal opticalelement drive unit is configured: to apply a first effective voltage tosaid first region; to apply a second effective voltage to said secondand sixth regions; to apply a third effective voltage to said third andseventh regions; to apply a fourth effective voltage to said fourth andeighth regions; to apply a fifth effective voltage to said fifth andninth regions, wherein an average of said second and fourth effectivevoltages and an average of said third and fifth effective voltages areequal to said first effective voltage; wherein said first effectivevoltage is V1, said second effective voltage is V1+Vα, said thirdeffective voltage is V1+Vβ, said fourth effective voltage is V1−Vα, andsaid fifth effective voltage is V1−Vβ, where V1 is a first referencevoltage value, Vα is a first voltage value, and Vβ is a second voltagevalue; and wherein an optimum value of said voltage Vβ is determinedwith said voltage Vα fixed to a predetermined value so that a qualityevaluation index of a reproduced signal from said optical recordingmedium is best improved, and an optimum value of said voltage Vα isdetermined with said voltage Vβ fixed to a predetermined value so thatsaid quality evaluation index is best improved.
 14. The opticalinformation recording/reproducing apparatus according to claim 1,wherein an optimum value of said voltage Vβ is determined with saidvoltage Vα fixed to a predetermined value so that a quality evaluationindex of a reproduced signal from said optical recording medium is bestimproved, and an optimum value of said voltage Vα is determined withsaid voltage Vβ fixed to a predetermined value so that said qualityevaluation index is best improved.
 15. The optical informationrecording/reproducing apparatus according to claim 1, wherein an optimumvalue of said voltage Vβ is determined with said voltage Vα fixed to apredetermined value so that a quality evaluation index of a reproducedsignal from said optical recording medium is best improved, and anoptimum value of said voltage Vα is determined with said voltage Vβfixed to a predetermined value so that said quality evaluation index isbest improved.
 16. The optical information recording/reproducing methodaccording to claim 7, wherein said controlling includes: determining anoptimum value of said voltage Vα with said voltage Vβ fixed at apredetermined value so that said quality evaluation index is bestimproved; and determining an optimum value of said voltage Vβ with saidvoltage Vα fixed at a predetermined value so that said qualityevaluation index is best improved.
 17. The optical informationrecording/reproducing method according to claim 7, wherein said liquidcrystal optical element further includes a second pattern electrodeprovided on another side of said liquid crystal polymer layer in saidoptical axis direction, positioned opposed to said first patternelectrode, and divided into a plurality of regions, wherein said secondpattern electrode includes: eighteenth and nineteenth regions providedapart from each other in an island structure at positions approximatelyin symmetry with respect to a straight line passing through said opticalaxis and directed in a predetermined direction; a twentieth regionprovided outside said eighteenth and nineteenth regions to surround saideighteenth and nineteenth regions; and twenty-first and twenty-secondregions provided outside said twentieth region approximately in symmetrywith said straight line on a same side of said eighteenth and nineteenthregions with respect to said straight line, wherein said driving furtherincludes: applying an effective voltage of V2 to said twentieth region;applying an effective voltage of V2−VΥ to said eighteenth andtwenty-second regions, and applying an effective voltage of V2+VΥ tosaid nineteenth and twenty-first regions, where V2 is a second referencevoltage value different from said first reference voltage value, and VΥis a third voltage value.
 18. The optical informationrecording/reproducing method according to claim 17, wherein saidcontrolling further includes determining an optimum value of saidvoltage VΥ with said voltages Vα and Vβ fixed at predetermined values sothat said quality evaluation index is best improved.
 19. The opticalinformation recording/reproducing method according to claim 7, whereinsaid liquid crystal optical element further includes a third patternelectrode provided on another side of said liquid crystal polymer layerin said optical axis direction, positioned opposed to said first patternelectrode, and divided into eighteenth and twenty-second regionsarranged in order from inside to outside to surround said optical axis,wherein said driving further includes: applying an effective voltage ofV3 to said nineteenth and twenty-first regions; applying an effectivevoltage of V3−Vδ to said twentieth region, and applying an effectivevoltage of V3+Vδ to said eighteenth and twenty-second regions, where V3is a third reference voltage value different from said first referencevoltage value, and Vδ is a fourth voltage value.
 20. The opticalinformation recording/reproducing method according to claim 7, whereinsaid quality evaluation index is one of an amplitude, jitter, PRSNR anderror rate of said reproduced signal.